feat(runtime): Lua ftplugin sets 'omnifunc', 'foldexpr' #32697

Problem:
- Many other ftplugin have defined 'omnifunc', but the Lua one doesn't
  define one, even though there is `vim.lua_omnifunc()`
- Users may want "stupid" completion to fix Lua config with
  `nvim --clean` in case they breaks it
- Nvim doesn't port Lua foldexpr from Vim

Solution:
- Set 'omnifunc' to 'v:lua.vim.lua_omnifunc' in ftplugin/lua.lua
- Set 'foldexpr' to use treesitter

runtime/doc/insert.txt

@@ -1506,6 +1506,11 @@ both major engines implemented element, even if this is not in standards it
 will be suggested. All other elements are not placed in suggestion list.
 
 
+LUA                                                     *ft-lua-omni*
+
+Lua |ftplugin| sets 'omnifunc' to |vim.lua_omnifunc()|.
+
+
 PHP							*ft-php-omni*
 
 Completion of PHP code requires a tags file for completion of data from

runtime/doc/news.txt

@@ -246,6 +246,10 @@ DEFAULTS
   • 'number', 'relativenumber', 'signcolumn', and 'foldcolumn' are disabled in
     |terminal| buffers. |terminal-config| shows how to change these defaults.
 
+• Options:
+  • Lua |ftplugin| sets 'omnifunc' to "v:lua.vim.lua_omnifunc".
+  • Lua |ftplugin| sets 'foldexpr' to "v:lua.vim.treesitter.foldexpr()".
+
 • Snippet:
   • `<Tab>` in Insert and Select mode maps to `vim.snippet.jump({ direction = 1 })`
     when a snippet is active and jumpable forwards.

runtime/ftplugin/lua.lua

@@ -1,4 +1,9 @@
 -- use treesitter over syntax
 vim.treesitter.start()
 
-vim.b.undo_ftplugin = (vim.b.undo_ftplugin or '') .. '\n call v:lua.vim.treesitter.stop()'
+vim.bo.omnifunc = 'v:lua.vim.lua_omnifunc'
+vim.wo[0][0].foldexpr = 'v:lua.vim.treesitter.foldexpr()'
+
+vim.b.undo_ftplugin = (vim.b.undo_ftplugin or '')
+  .. '\n call v:lua.vim.treesitter.stop()'
+  .. '\n setl omnifunc< foldexpr<'