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/_comment.lua

@@ -221,7 +221,7 @@ local function operator(mode)
   -- Used without arguments as part of expression mapping. Otherwise it is
   -- called as 'operatorfunc'.
   if mode == nil then
-    vim.o.operatorfunc = "v:lua.require'vim._comment'.operator"
+    vim.o.operatorfunc = operator
     return 'g@'
   end