Backport: Add support for pseudo elements for the block and its variations on theme.json

[scruffian]

What?

Closes WordPress/gutenberg#55359

Adds support for pseudo elements on the core/button block for ( ':hover', ':focus', ':focus-visible', ':active' ) at the theme.json level. This is also allowing the block's variations to control the same pseudo elements, so now we can style hover for the outline variation too.

Why?

This is needed ahead of WordPress/gutenberg#38277 since right now we are only supporting pseudo elements on links and button elements, we also want to support them at the block level. This PR brings them in alignment and opens the door to support things like :hover for other blocks that don't have an element (like group for example)

How?

By allowing the pseudo elements for the specific blocks in VALID_BLOCK_PSEUDO_SELECTORS, in a similar way we do for elements.

Testing Instructions

Add the following to your theme and check that it behaves as expected on the frontend:

"styles": {
	"blocks": {
		"core/button": {
			"color": {
				"background": "blue"
			},
			":hover": {
				"color": {
					"background": "green"
				}
			},
			":focus": {
				"color": {
					"background": "purple"
				}
			},
			"variations": {
				"outline": {
					":hover": {
						"color": {
							"background": "pink"
						}
					}
				}
			}
		}
	}
}

Select a button block, and in the Advanced section, change the element of the button to , using the HTML element option, and check everything still works.

Screenshots or screencast

Trac ticket: https://core.trac.wordpress.org/ticket/64263#ticket

---

This Pull Request is for code review only. Please keep all other discussion in the Trac ticket. Do not merge this Pull Request. See GitHub Pull Requests for Code Review in the Core Handbook for more details.

[github-actions]

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[github-actions]

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[carolinan]

Can we update the testing instructions to include changing the element of the button to <button>, using the HTML element option in the Advanced section.

[scruffian]

There was an issue with the backport - the nesting level of the fix was wrong. I pushed a fix for this.

[MaggieCabrera]

This is testing well for me, and it fits what we already have in GB. This should have been backported to 6.9, so let's bring it in for 7.0

[MaggieCabrera]

these need renaming I think

[github-actions]

A commit was made that fixes the Trac ticket referenced in the description of this pull request.

SVN changeset: 61607
GitHub commit: e9f34bd

This PR will be closed, but please confirm the accuracy of this and reopen if there is more work to be done.