[PHP 8.4] `mb_trim`, `mb_ltrim`, and `mb_rtrim` polyfills

[Ayesh]

RFC: Multibyte for trim function mb_trim, mb_ltrim and mb_rtrim
php.net: TBD
PHP.Watch: PHP 8.4: Mbstring: New mb_trim, mb_ltrim, and mb_rtrim functions

I think we can provide a polyfill for these functions. I created a small repo here with sample code that passes the test suite in php-src for this function. I'll happily submit a PR with it if you think that approach is good enough.

Thank you.

[stof]

PRs are happily accepted.

This should be added in 2 places:

• in the mbstring polyfill
• in the php 8.4 polyfill (which needs to be created as it does not exist yet), but only when mbstring is available (see how mb_str_split is handled in the PHP 7.4 polyfill)