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/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<'