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
f5714994bc
