Global Styles: Introduce filter for toggling stylesheet cache

[tyxla]

Currently, for wp_get_global_stylesheet() and wp_get_global_styles_svg_filters() we'll cache the merged stylesheet data per theme in a transient for a minute if:

• WP_DEBUG is not enabled;
• SCRIPT_DEBUG is not enabled;
• This is not a REST API request
• We're not in the admin

However, for huge multisite instances that leverage object caching, this can result in thousands of wp_cache_set() operations per minute, which can be quite intensive.

At the same time, retrieving the global stylesheet data and merging it, on a well-optimized server will usually take very little time (up to a few milliseconds).

As a consequence, we end up wanting to disable that caching, because it creates more problems than it solves. However, there is no good trigger for disabling just that cache.

I'd like to propose a filter that allows us to disable the global stylesheet cache. While it will still be enabled by default, disabling it will be easy when needed.

Trac ticket: https://core.trac.wordpress.org/ticket/55780

[tyxla]

Note: the change seems pretty trivial, which is why I didn't add a unit test. It feels like testing this would fall down to testing whether apply_filters() work, which is redundant.

I'm happy to add a test or two if y'all feel differently about this.

:wave: I wanted to cross-post that I commented at ​https://github.com/WordPress/gutenberg/pull/41201#issuecomment-1303111763 so it's not missed.

Cross-posting here as well:

Miguel has prepared ​https://github.com/WordPress/gutenberg/pull/45679 I think is very promising and should cover all cache invalidation we need. We only need to fix the automated tests.

Closing in favor of ​https://github.com/WordPress/gutenberg/pull/45679.

[pbearne]

Does the Gutenberg patch fix the old style stylesheet

Looking at the patches I don't think it does so I feel the patch is still valid.

Happy to add tests if needed