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"
						}
					}
				}
			}
		}
	}
}

Screenshots or screencast

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

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