Revert "Update function names for the public global styles API functions"

[oandregal]

Reverts #36907

What this PR fixes

When the plugin is active in WordPress 5.9, it doesn't override global styles WordPress core public API.

Context

In #36907 I renamed the public API of GS from gutengerg_* to wp_*), to make backport easier and make the functions available in all the WordPress versions the plugin supports. I hoped that we could introduce some filters at #36909 to modify their behavior from the plugin. However, we decided against that, and, as a side-effect, we don't have a way to overwrite the WordPress core functions when the plugin runs on WordPress 5.9.

Follow-ups post 5.9

We'd need to clarify how to best address the following (can be done post-5.9):

• What happens when/if we need to modify the behavior of the public GS API? In principle, we shouldn't update code that lives in lib/compat/5.9 after WordPress 5.9 has been released.

• What happens if we don't need to modify the behavior of those functions and the plugin minimum version is 5.9 (so the lib/compat/5.9 is removed? At that point, we can't remove the functions from the plugin as we may still need them to overwrite the core behavior.

Unless I'm missing something, it looks like the code for the public API of GS should not live in lib/compat.

[getdave]

I tested some of this whilst working on #36960 and it allowed me to make changes to the lib/class-wp-theme-json-gutenberg.php file where previously it would only load the WP Core version.

[oandregal]

I'm going to close this.

I ran this by some folks for thoughts (Riad, Jorge) and this is the consensus:

• Any global styles change aimed for 5.9 needs two PRs: one for the plugin and another for WordPress trunk.
• The changes to the plugin should be tested with WordPress 5.8, as they won't work with WordPress trunk (the plugin won't register the functions if WordPress core already did, see).
• If, and only if, there's a change that is not aimed for 5.9 we'll look into overriding the global styles code that lives in lib/compat/5.9.

The rationale for this is that we want to remove as much code from the plugin as possible and this is another step in that direction for global styles.

cc some folks I know contributed to the GS code for awareness @getdave @noisysocks @aaronrobertshaw @andrewserong @scruffian @fabiankaegy @Mamaduka @carolinan @ajlende @aristath @walbo

[oandregal]

Draft PR to move the global styles related code under lib/compat/wordpress-5.9 folder #36978

[oandregal]

#36978 is ready for review.