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/lua/vim/treesitter/dev.lua

@@ -639,7 +639,7 @@ function M.edit_query(lang)
   local query_buf = api.nvim_win_get_buf(query_win)
 
   vim.b[buf].dev_edit = query_win
-  vim.bo[query_buf].omnifunc = 'v:lua.vim.treesitter.query.omnifunc'
+  vim.bo[query_buf].omnifunc = vim.treesitter.query.omnifunc
   set_dev_options(query_win, query_buf)
 
   -- Note that omnifunc guesses the language based on the containing folder,