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/man.lua

@@ -768,7 +768,7 @@ function M.open_page(count, smods, args)
   end
   local buf = api.nvim_get_current_buf()
   local save_tfu = vim.bo[buf].tagfunc
-  vim.bo[buf].tagfunc = "v:lua.require'man'.goto_tag"
+  vim.bo[buf].tagfunc = M.goto_tag
 
   local target = ('%s(%s)'):format(name, sect)