Add g:loaded_nerdfont, play nice with other plugins

Allow other plugins or conditional code in vimrc detect that
vim-nerdfont is available without having to check &rtp etc.

Useful for plugins that automatically use vim-devicons to optionally use
vim-nerdfont if installed, e.g. https://github.com/Donaldttt/fuzzyy

Author: mmrwoods

plugin/nerdfont.vim

@@ -0,0 +1,5 @@
+" Allow easy detection of nerdfont plugin loaded
+if exists("g:loaded_nerdfont")
+  finish
+endif
+let g:loaded_nerdfont = 1