Prevent redundant block style variation CSS

[aaronrobertshaw]

This ticket tracks the backporting of PHP files for the following Gutenberg fix:
​https://github.com/WordPress/gutenberg/pull/62465

These changes prevent duplicate and redundant CSS being generated for block style variations.

This PR backports the PHP updates from: ​https://github.com/WordPress/gutenberg/pull/62465

The changes prevent redundant or duplicate CSS being generated for block style variations.

### Test Instructions

1. Register a custom block style variation e.g. through a theme.json partial: /styles/partial.json

{
	"$schema": "https://schemas.wp.org/trunk/theme.json",
	"version": 2,
	"title": "Custom",
	"slug": "custom",
	"blockTypes": [ "core/group" ],
	"styles": {
		"color": {
			"background": "aliceblue"
		}
	}
}

2. In both editors add a group block and apply the Custom block style variation.
3. Inspect the group block and confirm there is only a single is-style-custom-* css class matched
4. Save the page or post and confirm the same on the frontend

Trac ticket: https://core.trac.wordpress.org/ticket/61443

I noticed while creating this backport of ​https://github.com/WordPress/gutenberg/pull/62465 that there's a difference in signature between core and Gutenberg for get_block_nodes.

Before this change:

Core: private static function get_block_nodes( $theme_json )
Gutenberg: private static function get_block_nodes( $theme_json, $selectors = array() )

I'll try and sort this out in time for beta 3.

The PR modifying the function signature in Gutenberg was ​https://github.com/WordPress/gutenberg/pull/41217.

From ​https://github.com/WordPress/gutenberg/pull/41217#issuecomment-1354426105 it looks like it the fact the PR wasn't backported was already known however perhaps not that "everything will be fine" as suggested after ​https://github.com/WordPress/gutenberg/pull/46579.

As far as I can tell, I don't think there'll be any harm in updating get_block_nodes function to match Gutenberg's version.

I've made the required updates from ​https://github.com/WordPress/gutenberg/pull/41217.

Flagging this as ready for review once the Gutenberg PR lands.

This PR is testing well for me. For blocks with the block style variation applied, I'm seeing the following, related classnames in the class attribute in both the editor and frontend:

is-style-custom is-style-custom-b321ad1e-3e38-4a3c-af91-ef82f0c9855d

I'm seeing the following, related classnames in the class attribute in both the editor and frontend

For the moment, this is expected. The is-style-custom classname is what's applied by the editor and is hooked onto by the block support to generate the class specific to this instance or application of the block style, is-style-custom--<hash>.

Just a quick note that applying this patch on top of the latest trunk (after the custom CSS fix) still shows everything working as expected 👍

[oandregal]

In 58422:

Global styles: prevent duplicate CSS for block style variations.

Props aaronrobertshaw, mukesh27, ramonopoly, isabel_brison, oandregal.
Fixes #61443.

Committed at https://core.trac.wordpress.org/changeset/58422