feat(ftplugin): set Lua 'omnifunc' to vim.lua_omnifunc #32491

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

Solution:
Set 'omnifunc' to 'v:lua.vim.lua_omnifunc' in ftplugin/lua.lua

runtime/doc/insert.txt

@@ -1502,6 +1502,11 @@ 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

@@ -239,6 +239,9 @@ DEFAULTS
     • |[b|, |]b|, |[B|, |]B| navigate through the |buffer-list|
     • |[<Space>|, |]<Space>| add an empty line above and below the cursor
 
+• Options:
+  • Lua |ftplugin| sets |'omnifunc'| to `"v:lua.vim.lua_omnifunc"`.
+
 • 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 +1,7 @@
 -- 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.b.undo_ftplugin = (vim.b.undo_ftplugin or '')
+  .. '\n call v:lua.vim.treesitter.stop() \n setl omnifunc<'