Editor: Use a non-persistent object cache instead of transient in wp_get_global_stylesheet().
This changeset is part of a greater effort to enhance the caching strategy for theme.json based data. Similar to [55138], [55148], and [55155], the cache is currently ignored when WP_DEBUG is on to avoid interrupting the theme developer's workflow.
Props spacedmonkey, oandregal, flixos90, ajlende, hellofromtonya.
Fixes #57568.