feat(options): lua closure/function options

Problem:
Cannot assign Lua functions/closures to "func" ('completefunc',
'tagfun', …) or "expr" ('foldexpr', 'indentexpr', …) options.

Solution:
- Store "func"/"expr" options as `Callback` instead of string.
- Delete oceans of copy-pasted code.
- BREAKING: LuaRef returned via RPC/Vimscript is now represented as
  `"<Lua N: file:line>"` (like what `:map` shows) instead of `nil`.
- Note: `man.vim` still uses `v:lua` string, bc it's a vimscript ftplugin.

Helped-by: Lewis Russell <lewis6991@gmail.com>

runtime/ftplugin/query.lua

@@ -11,7 +11,7 @@ end
 vim.treesitter.start()
 
 -- set omnifunc
-vim.bo.omnifunc = 'v:lua.vim.treesitter.query.omnifunc'
+vim.bo.omnifunc = vim.treesitter.query.omnifunc
 
 vim.api.nvim_set_option_value('iskeyword', '.', { scope = 'local', operation = 'append' })