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/_core/defaults.lua

@@ -446,14 +446,12 @@ do
 
     -- Add empty lines
     vim.keymap.set('n', '[<Space>', function()
-      -- TODO: update once it is possible to assign a Lua function to options #25672
-      vim.go.operatorfunc = "v:lua.require'vim._core.util'.space_above"
+      vim.go.operatorfunc = require('vim._core.util').space_above
       return 'g@l'
     end, { expr = true, desc = 'Add empty line above cursor' })
 
     vim.keymap.set('n', ']<Space>', function()
-      -- TODO: update once it is possible to assign a Lua function to options #25672
-      vim.go.operatorfunc = "v:lua.require'vim._core.util'.space_below"
+      vim.go.operatorfunc = require('vim._core.util').space_below
       return 'g@l'
     end, { expr = true, desc = 'Add empty line below cursor' })
   end

runtime/lua/vim/_core/util.lua

@@ -3,7 +3,6 @@
 local M = {}
 
 --- Adds one or more blank lines above or below the cursor.
--- TODO: move to _core/defaults.lua once it is possible to assign a Lua function to options #25672
 --- @param above? boolean Place blank line(s) above the cursor
 local function add_blank(above)
   local offset = above and 1 or 0
@@ -12,12 +11,10 @@ local function add_blank(above)
   vim.api.nvim_buf_set_lines(0, linenr - offset, linenr - offset, true, repeated)
 end
 
--- TODO: move to _core/defaults.lua once it is possible to assign a Lua function to options #25672
 function M.space_above()
   add_blank(true)
 end
 
--- TODO: move to _core/defaults.lua once it is possible to assign a Lua function to options #25672
 function M.space_below()
   add_blank()
 end