viml: embed Lua syntax highlighting [skip ci] (#14213)

runtime/doc/vim_diff.txt

@@ -70,6 +70,8 @@ the differences.
 - |matchit| plugin is enabled. To disable it in your config: >
     :let loaded_matchit = 1
 
+- |g:vimsyn_embed| defaults to "l" to enable Lua highlighting
+
 ==============================================================================
 3. New Features						       *nvim-features*
 

runtime/syntax/vim.vim

@@ -616,7 +616,7 @@ syn region	vimGlobal	matchgroup=Statement start='\<v\%[global]!\=/' skip='\\.' e
 "   g:vimsyn_embed =~# 'r' : embed ruby
 "   g:vimsyn_embed =~# 't' : embed tcl
 if !exists("g:vimsyn_embed")
- let g:vimsyn_embed= 0
+ let g:vimsyn_embed = 'l'
 endif
 
 " [-- lua --] {{{3