feat: add js_trim() and mb_trim() compat

[USERSATOSHI]

PHP’s trim() function, by default, only strips a limited set of ASCII whitespace characters, and mb_trim(), introduced in PHP 8.4, does not behave identically to JavaScript’s String.prototype.trim().

This PR implements js_trim(), a PHP function that replicates JavaScript’s String.prototype.trim() behavior.

It works by defining a set of $js_trimmables characters, which are passed to mb_trim() with UTF-8 encoding.

In addition, this PR adds a polyfill for mb_trim() in compat.php to support PHP versions below 8.4 with unit tests for both js_trim() and mb_trim()

Trac ticket: https://core.trac.wordpress.org/ticket/63804

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