I18N: Change how WP_Textdomain_Registry stores the default languages path.
WP_Textdomain_Registry was introduced in [53874] to store text domains and their language directory paths, addressing issues with just-in-time loading of textdomains when using locale switching and when usingload_*_textdomain() functions.
Said change has inadvertently caused a performance regression exactly when usingload_*_textdomain(), which still often is the case, where the cached information was not further used or even overridden.
This change addresses that issue by storing the default languages paths in a separate way, while at the same time making WP_Textdomain_Registry easier to maintain and adding new tests to catch future regressions.
Props flixos90, spacedmonkey, ocean90, SergeyBiryukov, costdev.
Fixes #39210.