fix(lua): vim.diff is nil in uv.new_work() thread #34909

Problem:
The "gitsigns" plugin runs `vim.diff` in a thread (`uv.new_work`), but
`vim.diff` is nil in that context:

    Lua callback:
    …/gitsigns.nvim/lua/gitsigns/diff_int.lua:30: bad argument #1 to 'decode' (string expected, got nil)
    stack traceback:
      [C]: in function 'decode'
      …/gitsigns.nvim/lua/gitsigns/diff_int.lua:30: in function <…/gitsigns.nvim/lua/gitsigns/diff_int.lua:29>
    Luv thread:
    …/gitsigns.nvim/lua/gitsigns/diff_int.lua:63: attempt to call field 'diff' (a nil value)

Solution:
Revert the `stdlib.c` change (set `vim.diff` instead of `vim._diff`).

runtime/lua/vim/text.lua

@@ -73,7 +73,8 @@ local M = {}
 ---@param opts? vim.text.diff.Opts
 ---@return string|integer[][]? # See {opts.result_type}. `nil` if {opts.on_hunk} is given.
 function M.diff(...)
-  return vim._diff(...)
+  ---@diagnostic disable-next-line: deprecated
+  return vim.diff(...)
 end
 
 local alphabet = '0123456789ABCDEF'