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/_meta/misc.lua

@@ -14,3 +14,11 @@
 --- @param ... any
 --- @return any
 function vim.call(func, ...) end
+
+--- Renamed to `vim.text.diff`, remove at Nvim 1.0
+---@deprecated
+---@param a string First string to compare
+---@param b string Second string to compare
+---@param opts? vim.text.diff.Opts
+---@return string|integer[][]? # See {opts.result_type}. `nil` if {opts.on_hunk} is given.
+function vim.diff(a, b, opts) end