Add inline diff annotations with gutter symbols

[SRWieZ]

Diff annotations

Implements inline diff annotations using comment-based syntax with full range support:

• [code! insert], [code! add], [code! ++] for additions
• [code! remove], [code! del], [code! delete], [code! --] for deletions
• Gutter diff symbols: + for insert lines, - for remove lines

Fixes #117

CSS variables

I didn't find where CSS variables were added for https://phiki.dev/multi-themes and https://phiki.dev/commonmark#sample-css-2

So I added a function to do the following:

• Automatic theme color extraction from markup.inserted/markup.deleted
• CSS variables: --phiki-diff-insert-bg/fg, --phiki-diff-remove-bg/fg
• Multi-theme support with prefixed variables (--phiki-light/dark-*)

Screenshots

[ryangjchandler]

Dude, this is awesome! I'll get around to reviewing this very soon, a little busy with Laravel Forge right now.

Regardless, thank you. Excited to check this out.

[ryangjchandler]

I haven't forgotten about this @SRWieZ – on my list of things to look at!

[SRWieZ]

Don't worry, take your time!

I have two other PRs in mind after this one, but there are probably things to improve here. I would prefer to have your review and fixes on this one before starting on the other two 🙂