Update Theme JSON $schema to account for pseudo selector support

[getdave]

What?

In #41786 we added pseudo selector support to Theme JSON. We now need to update the schema for Theme JSON so we get accurate feedback in code editors such as Visual Studio Code.

When working on this PR you might like to reference: https://json-schema.org/understanding-json-schema/

Why?

We need good develop experience

How?

Update the schema for elements both at the top level and inside blocks to know about the valid pseudo selectors - :hover, :focus, and :active on the link element only.

Note that this introduces some duplication in under the stylesElementsPropertiesComplete.properties.link.allOf[1].properties object where

"border": {},
"color": {},
"spacing": {},
"typography": {},

is effectively duplicating stylesPropertiesComplete.

@adamziel and I spent some time trying to work out a way to DRY this up, but the way additionalProperties locks things down for extension seems to preclude the possibility of achieving this.

In the future as more elements start to allow pseudo elements we may need to DRY the schema up more but for now this is good enough.

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 some pseudo selector rules under elements.link and {{BLOCK_NAME}}.elements.link and check your IDE doesn't complain to you.
4. Add some invalid pseudo selectors and check it does complain.
5. Add some pseudo rules under an element which is not link and check it complains.

Screenshots or screencast

[mikachan]

Ran through the testing instructions and this works well for me 👍