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>

scripts/linterrcodes.lua

@@ -49,7 +49,7 @@ local dup_allowed = {
   E509 = 2,
   E5101 = 2,
   E5102 = 2,
-  E5108 = 5,
+  E5108 = 6,
   E5111 = 2,
   E513 = 2,
   E521 = 2,