Update global styles public API

[oandregal]

This simplifies the public API of the newly introduced methods in WordPress 5.9. It's based on feedback received as per #36556 and future improvements (such as having multiple theme.json files).

How to test

• Verify that tests still pass.
• Load the front and editors (post, site) and verify that it works as expected (presets and styles are loaded, etc).

[skorasaurus]

Maybe worth mentioning in a separate issue; but do we also want to keep the gutenberg_ prefix at the beginning of the functions?

[mtias]

All of those should be removed with the build for core.

[mcsf]

Edit: disregard my comment, as this only applies to block-library.

[…] but do we also want to keep the gutenberg_ prefix at the beginning of the functions?

gutenberg/tools/webpack/blocks.js

Lines 123 to 129 in 80e3447

	// Prepend the Gutenberg prefix, substituting any
	// other core prefix (e.g. "wp_").
	return result.replace(
	new RegExp( functionName, 'g' ),
	( match ) =>
	'gutenberg_' +
	match.replace( /^wp_/, '' )

[mtias]

Do we want function_exists checks for core?

[oandregal]

We don't need them as the current naming stands, but I discussed this with Miguel and I'm going to check how do we want this public API to operate (see). I'll post as a comment any follow-up PR.

[mcsf]

Per our private conversation, this PR looks good in terms of API changes. As for the naming of the function so as to operate across plugin and core, that's another question. The two main opposing methods are:

1. Keeping Core functions as the canonical ones, loaded first, with filters in them that allow the plugin to short-circuit them.
2. Keeping plugin functions as the canonical ones, loaded first, then wrapping the Core definitions with if ( ! function_exists guards.

[oandregal]

I've leveled up my understanding of how lib/compat/* code should work, and I'm proposing a follow-up to this at #36907

[oandregal]

The second follow-up I thought we need is introducing short-circuit filters to be able to add new behavior from the plugin #36909

[oandregal]

We decided against the filters. Rationale at #36909 (comment)