Add ability for custom handlers to extend builtin

# Details

Currently custom handlers can only replace the logic of builtin
handlers. Meaning adding one feature to the markdown handler is
not possible without replacing all of the existing logic.

Allow users to specify a new field in each handler called extends,
which defaults to false. When set to true both the user defined
handler as well as the builtin one will run.

This can lead to contradictions in behavior, this is not handled
in any way and I don't really think there is a good way to do this.
Ideally these custom handlers offer functionality that does not overlap
with any of the existing functionality.

Make some other minor changes as well such as where the disable latex
logic lives.

Update demos and update script to handle setting the handler interface
in the README.

Author: MeanderingProgrammer

README.md

@@ -24,7 +24,7 @@ Plugin to improve viewing Markdown files in Neovim
 - Basic support for `LaTeX` if `latex` parser and `pylatexenc` are installed
 - Disable rendering when file is larger than provided value
 - Support for [callouts](https://github.com/orgs/community/discussions/16925)
-- Support custom handlers which are ran identically to native handlers
+- Support custom handlers which are ran identically to builtin handlers
 
 # Limitations
 
@@ -260,10 +260,14 @@ vim.treesitter.language.register('markdown', 'vimwiki')
 # Custom Handlers
 
 Custom handlers allow users to integrate custom rendering for either unsupported
-languages or to override the native implementations. This can also be used to
-disable a native language, as custom handlers have priority.
+languages or to override / extend the builtin implementations.
 
-For example disabling the `LaTeX` handler can be done with:
+This can also be used to disable a builtin handler, by not specifying the `extends`
+field and leaving the implementation blank. Though this has little benefit and
+can be accomplished in other ways such as setting `{ latex_enabled = false }`
+for `LaTeX`.
+
+Still as an example disabling the `LaTeX` handler can be done with:
 
 ```lua
 require('render-markdown').setup({
@@ -278,6 +282,7 @@ Each handler must conform to the following interface:
 ```lua
 ---@class render.md.Handler
 ---@field public render fun(namespace: integer, root: TSNode, buf: integer)
+---@field public extends? boolean
 ```
 
 The `render` function parameters are:
@@ -286,15 +291,18 @@ The `render` function parameters are:
 - `root`: The root treesitter node for the specified language
 - `buf`: The buffer containing the root node
 
-Custom handlers are ran identically to native ones, so by writing custom `extmark`s
+The `extends` parameter defines whether the builtin handler should still be run in
+conjunction with this one. Defaults to `false`.
+
+Custom handlers are ran identically to builtin ones, so by writing custom `extmark`s
 (see :h nvim_buf_set_extmark()) to the provided `namespace` this plugin will handle
 clearing the `extmark`s on mode changes as well as re-calling the `render` function
 when needed.
 
 This is a high level interface, as such creating, parsing, and iterating through
 a treesitter query is entirely up to the user if the functionality they want needs
 this. We do not provide any convenience functions, but you are more than welcome
-to use patterns from the native handlers.
+to use patterns from the builtin handlers.
 
 ## More Complex Example
 

demo/box_dash_quote.gif

@@ -1,4 +1,4 @@
-*render-markdown.txt*           For 0.10.0           Last change: 2024 June 17
+*render-markdown.txt*           For 0.10.0           Last change: 2024 June 19
 
 ==============================================================================
 Table of Contents                          *render-markdown-table-of-contents*
@@ -51,7 +51,7 @@ Plugin to improve viewing Markdown files in Neovim
 - Basic support for `LaTeX` if `latex` parser and `pylatexenc` are installed
 - Disable rendering when file is larger than provided value
 - Support for callouts <https://github.com/orgs/community/discussions/16925>
-- Support custom handlers which are ran identically to native handlers
+- Support custom handlers which are ran identically to builtin handlers
 
 
 ==============================================================================
@@ -303,10 +303,14 @@ the `filetype` of `markdown` files there are additional setup steps.
 9. Custom Handlers                           *render-markdown-custom-handlers*
 
 Custom handlers allow users to integrate custom rendering for either
-unsupported languages or to override the native implementations. This can also
-be used to disable a native language, as custom handlers have priority.
+unsupported languages or to override / extend the builtin implementations.
 
-For example disabling the `LaTeX` handler can be done with:
+This can also be used to disable a builtin handler, by not specifying the
+`extends` field and leaving the implementation blank. Though this has little
+benefit and can be accomplished in other ways such as setting `{ latex_enabled
+= false }` for `LaTeX`.
+
+Still as an example disabling the `LaTeX` handler can be done with:
 
 >lua
     require('render-markdown').setup({
@@ -321,6 +325,7 @@ Each handler must conform to the following interface:
 >lua
     ---@class render.md.Handler
     ---@field public render fun(namespace: integer, root: TSNode, buf: integer)
+    ---@field public extends? boolean
 <
 
 The `render` function parameters are:
@@ -329,15 +334,18 @@ The `render` function parameters are:
 - `root`: The root treesitter node for the specified language
 - `buf`: The buffer containing the root node
 
-Custom handlers are ran identically to native ones, so by writing custom
+The `extends` parameter defines whether the builtin handler should still be run
+in conjunction with this one. Defaults to `false`.
+
+Custom handlers are ran identically to builtin ones, so by writing custom
 `extmark`s (see :h nvim_buf_set_extmark()) to the provided `namespace` this
 plugin will handle clearing the `extmark`s on mode changes as well as
 re-calling the `render` function when needed.
 
 This is a high level interface, as such creating, parsing, and iterating
 through a treesitter query is entirely up to the user if the functionality they
 want needs this. We do not provide any convenience functions, but you are more
-than welcome to use patterns from the native handlers.
+than welcome to use patterns from the builtin handlers.
 
 
 MORE COMPLEX EXAMPLE    *render-markdown-custom-handlers-more-complex-example*

demo/callout.gif

@@ -37,22 +37,21 @@ demo file rows content:
   rm {{file}}.cast
 
 update:
+  # Updates types.lua & README.md
   python -Wignore scripts/update.py
-
-gen-doc:
-  # https://github.com/kdheepak/panvimdoc
   # https://pandoc.org/
+  # https://github.com/kdheepak/panvimdoc
   ../../open-source/panvimdoc/panvimdoc.sh \
     --project-name render-markdown \
     --input-file README.md \
     --vim-version 0.10.0
 
 [private]
-gen-file-text:
+gen-large-file-text:
   #!/usr/bin/env python
   for i in range(100_000):
     level = "#" * ((i % 6) + 1)
     print(f"{level} Title {i}\n")
 
 gen-large-file:
-  just gen-file-text > large.md
+  just gen-large-file-text > large.md

demo/heading_code.gif

@@ -15,6 +15,9 @@ local M = {}
 ---@param root TSNode
 ---@param buf integer
 M.render = function(namespace, root, buf)
+    if not state.config.latex_enabled then
+        return
+    end
     local converter = state.config.latex_converter
     if vim.fn.executable(converter) ~= 1 then
         logger.debug('Executable not found: ' .. converter)

demo/latex.gif

@@ -36,6 +36,7 @@ local M = {}
 
 ---@class render.md.Handler
 ---@field public render fun(namespace: integer, root: TSNode, buf: integer)
+---@field public extends? boolean
 
 ---@class render.md.WindowOption
 ---@field public default any