[PHP 8.4] `mb_ucfirst()` and `mb_lcfirst()` polyfills

* [PHP RFC: Multibyte for ucfirst, lcfirst functions, mb_ucfirst mb_lcfirst](https://wiki.php.net/rfc/mb_ucfirst)
* [New `mb_ucfirst` and `mb_lcfirst` functions - PHP 8.4 • PHP.Watch](https://php.watch/versions/8.4/mb_ucfirst-mb_ucfirst)

These functions have been merged at https://github.com/php/php-src/pull/13161.