This introduces a more lightweight library for loading `.mo` translation files which offers increased speed and lower memory usage.
It also supports loading multiple locales at the same time, which makes locale switching faster too.
For plugins interacting with the `$l10n` global variable in core, a shim is added to retain backward compatibility with the existing `pomo` library.
In addition to that, this library supports translations contained in PHP files, avoiding a binary file format and leveraging OPCache if available.
If an `.mo` translation file has a corresponding `.l10n.php` file, the latter will be loaded instead.
This behavior can be adjusted using the new `translation_file_format` and `load_translation_file` filters.
PHP translation files will be typically created by downloading language packs, but can also be generated by plugins.
See https://make.wordpress.org/core/2023/11/08/merging-performant-translations-into-core/ for more context.
Props dd32, swissspidy, flixos90, joemcgill, westonruter, akirk, SergeyBiryukov.
Fixes#59656.
Built from https://develop.svn.wordpress.org/trunk@57337
git-svn-id: http://core.svn.wordpress.org/trunk@56843 1a063a9b-81f0-0310-95a4-ce76da25c4cd
