Trac ticket https://core.trac.wordpress.org/ticket/58462
Backports WordPress/gutenberg#49807

What?

This PR makes sure unregistered block style variations are ignored before processing any data coming from theme.json.

Why?

• The current state prevents registered style variations from working in the editor when an unregistered style variation is found in a theme.json dataset.
• It also enqueues CSS rules to the front-end that don't have a selector, so they are invalid.

How?

By making sure the style variation is not added to the allowed list, unless it's registered for that particular block.

Testing Instructions

• Make sure your environment logs the errors to the screen by setting WP_DEBUG to true.
• Add the following two variations to the theme.json of TwentyTwentyThree:

{
  "styles": {
    "blocks": {
      "core/quote": {
        "variations": {
          "plain": { "color": { "background": "indigo", "text": "gold" } },
          "unregistered": { "color": { "background": "red", "text": "blue" } }
        } 
      }
    }
  }
}

• Create a post that contains a quote and style it with the plain variation.

The expected result is that:

• The styles are applied to quote blocks with plain style variation in editors and front.
• The wp-block-quote-inline-css stylesheet in the front-end doesn't have a style rule such as { background-color: red; color: blue} (note the lack of CSS selector).
• There is no error reported in class-wp-theme-json.php for the editors or front. In trunk, you'll see something like this:

Verify that upon updating the colors of the style variation via the global styles sidebar (blocks > quote > style variation plain), the user-provided colors are rendered everywhere (front & editors).

Commit message

Ignore unregistered block style variations from `theme.json`.

This PR makes sure unregistered block style variations declared via `theme.json` are ignored. It fixes an issue by style variations don't work in the editor and CSS rules without a selector are output to the front-end.

Props isabel_brison.
Fixes #58462.