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/lsp/client.lua

@@ -1448,10 +1448,10 @@ end
 --- Reset defaults set by `set_defaults`.
 --- Must only be called if the last client attached to a buffer exits.
 local function reset_defaults(bufnr)
-  if vim.bo[bufnr].tagfunc == 'v:lua.vim.lsp.tagfunc' then
+  if vim.bo[bufnr].tagfunc == vim.lsp.tagfunc then
     vim.bo[bufnr].tagfunc = nil
   end
-  if vim.bo[bufnr].omnifunc == 'v:lua.vim.lsp.omnifunc' then
+  if vim.bo[bufnr].omnifunc == vim.lsp.omnifunc then
     vim.bo[bufnr].omnifunc = nil
   end
   if vim.bo[bufnr].formatexpr == 'v:lua.vim.lsp.formatexpr()' then