Add missing pseudo-selectors to theme.json schema and documentation

[t-hamano]

Related core ticket: Changeset 55121 – WordPress Trac
Related issue: #49013

What?

This PR adds the missing pseudo-selectors to the JSON schema and developer documentation.

Testing Instructions

Create a JSON file that references this PR with the $schema property:

{
	"$schema": "https://raw.githubusercontent.com/WordPress/gutenberg/fix/schema-for-pseudo-classes/schemas/json/theme.json",
	"version": 2,
	"styles": {
		"elements": {
			"link": {
			},
			"button": {
			}
		},
		"blocks": {
			"core/button": {
				"elements": {
					"button": {
					},
					"link": {
					}
				}
			}
		}
	}
}

Confirm that a total of six pseudo-classes appear as property candidates directly under elements.link and elements.button.

[t-hamano]

I have found that a similar schema fix was backported to wp/6.2 branch in #48948. I don't know if it is possible to backport it now, but I will give it a backport label just in case.

[ajlende]

Looks good to me 👍

I see :link and :any-link in the list of VALID_ELEMENT_PSEUDO_SELECTORS.

[Mamaduka]

I just cherry-picked this PR to the wp/6.2 branch to get it included in the next release: 6f05e23