Update Theme JSON $schema to allow pseudo selectors on button property

[stephymiehle]

What?

Provide better IDE feedback to show that the pseudo selectors :hover, :focus, and :active are allowed on the button property

See also:

• Support pseudo selectors on elements in theme json #41786: Pseudo selector support enabled
• Update Theme JSON $schema to account for pseudo selector support #41936: Pseudo selectors allowed on link properties

Why?

Improving the developer experience – anecdotally, I was trying to implement a button hover style in a full-site theme but wasn't certain that I was doing so the right way since the schema was disallowing my solution

How?

Update the schema for top-level and interior blocks to allow the valid pseudo selectors :hover, :focus, and :active on the button property

As mentioned in #41936, this adds some code duplication and is not the DRYest solution. If this were implemented for all of the other properties in the same manner, we'd be extremely redundant. However, links and buttons are both very standard properties to support pseudo selector states; I feel that it's fair to add a small bit of duplication to support such a common use.

Testing Instructions

1. Activate a Theme which you can modify locally.
2. Open the Theme's theme.json file and update the $schema to point to your local Gutenberg version do the schema file (e.g. "$schema": "file:///Users/{{YOUR_USER}}/Sites/gutenberg/schemas/json/theme.json").
3. Add allowed pseudo selector rules (:hover, :focus, and/or :active) under elements.button and {{BLOCK_NAME}}.elements.button and make sure your editor does not show any warnings or errors.
4. Add disallowed pseudo selector rules (such as any applied to :empty) and make sure your editor flags them as disallowed.
5. Add allowed pseudo selector rules under an unsupported element (such as h2) and make sure your editor flags them as disallowed.

Shamelessly adapted from #41936

Screenshots or screencast

Before

Common button pseudo selectors are disallowed even though they are functional and follow the same format/pattern as link:

After

Allowed pseudo selectors no longer show up as errors, but disallowed ones still do:

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[getdave]

This looks good to me. Matches the work we did for link element.

At some point we can DRY this up, but it's not necessary at this point.

Thank you for taking the time to contribute - it's much appreciated 🙇

[Mamaduka]

The failing e2e test isn't related to this change (See #44350), so I'm going to merge the PR.

Thanks for contributing, @blindingstars!

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Kudos!