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>

test/functional/api/vim_spec.lua

@@ -1898,6 +1898,11 @@ describe('API', function()
       ok(not api.nvim_get_option_value('equalalways', {}))
     end)
 
+    it('Lua funcref RPC value is a `:map`-style "<Lua …>" string', function()
+      exec_lua('vim.o.operatorfunc = function() end')
+      matches('^<Lua %d+.*>$', api.nvim_get_option_value('operatorfunc', {}))
+    end)
+
     it('works to get global value of local options', function()
       eq(false, api.nvim_get_option_value('lisp', {}))
       eq(8, api.nvim_get_option_value('shiftwidth', {}))