Global styles: block background UI controls

[ramonjd]

Syncing source code changes from WordPress/gutenberg#60100

Enables background images for supported blocks in global styles.

Testing

This PR adds the backend functionality only, which means the editor UI isn't yet available.

To test, define some background images for blocks. Here's an example using some of the images in src/wp-content/themes/twentytwentyfour/

{
	"$schema": "../../schemas/json/theme.json",
	"version": 3,
	"settings": {
		"appearanceTools": true
	},
	"styles": {
		"blocks": {
			"core/verse": {
				"background": {
					"backgroundImage": {
						"url": "file:./assets/images/art-gallery.webp"
					}
				}
			},
			"core/quote": {
				"background": {
					"backgroundImage": {
						"url": "file:./assets/images/building-exterior.webp"
					}
				}
			},
			"core/pullquote": {
				"background": {
					"backgroundImage": {
						"url": "file:./assets/images/hotel-facade.webp"
					}
				}
			},
			"core/post-content": {
				"background": {
					"backgroundImage": {
						"url": "file:./assets/images/windows.webp"
					}
				}
			}
		}
	}
}

Here is some test HTML to create a test post:

<!-- wp:verse {"style":{"elements":{"link":{"color":{"text":"var:preset|color|base-2"}}},"spacing":{"padding":{"top":"var:preset|spacing|60","bottom":"var:preset|spacing|60","left":"var:preset|spacing|60","right":"var:preset|spacing|60"}}},"textColor":"base-2","fontSize":"x-large"} -->
<pre class="wp-block-verse has-base-2-color has-text-color has-link-color has-x-large-font-size" style="padding-top:var(--wp--preset--spacing--60);padding-right:var(--wp--preset--spacing--60);padding-bottom:var(--wp--preset--spacing--60);padding-left:var(--wp--preset--spacing--60)">Verse</pre>
<!-- /wp:verse -->

<!-- wp:pullquote {"style":{"elements":{"link":{"color":{"text":"var:preset|color|base-2"}}}},"textColor":"base-2","fontSize":"x-large"} -->
<figure class="wp-block-pullquote has-base-2-color has-text-color has-link-color has-x-large-font-size"><blockquote><p>Pullquote</p></blockquote></figure>
<!-- /wp:pullquote -->

<!-- wp:quote -->
<blockquote class="wp-block-quote"><!-- wp:paragraph {"style":{"spacing":{"padding":{"top":"var:preset|spacing|60","bottom":"var:preset|spacing|60","left":"var:preset|spacing|60","right":"var:preset|spacing|60"}},"elements":{"link":{"color":{"text":"var:preset|color|base-2"}}}},"textColor":"base-2","fontSize":"x-large"} -->
<p class="has-base-2-color has-text-color has-link-color has-x-large-font-size" style="padding-top:var(--wp--preset--spacing--60);padding-right:var(--wp--preset--spacing--60);padding-bottom:var(--wp--preset--spacing--60);padding-left:var(--wp--preset--spacing--60)">Quote</p>
<!-- /wp:paragraph --></blockquote>
<!-- /wp:quote -->

The editor and frontend should display the background images:

Trac ticket: https://core.trac.wordpress.org/ticket/61588

---

This Pull Request is for code review only. Please keep all other discussion in the Trac ticket. Do not merge this Pull Request. See GitHub Pull Requests for Code Review in the Core Handbook for more details.

[github-actions]

Test using WordPress Playground

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Test this pull request with WordPress Playground.

[github-actions]

The following accounts have interacted with this PR and/or linked issues. I will continue to update these lists as activity occurs. You can also manually ask me to refresh this list by adding the props-bot label.

Core Committers: Use this line as a base for the props when committing in SVN:

Props ramonopoly, aaronrobertshaw, noisysocks.

To understand the WordPress project's expectations around crediting contributors, please review the Contributor Attribution page in the Core Handbook.

[aaronrobertshaw]

Thanks for putting this together @ramonjd 👍

✅ Theme.json defined background images display in editors and frontend
✅ Code changes match GB PR

Editor	Frontend

I left a few minor nits as inline comments.

[aaronrobertshaw]

Thanks for adding the @ticket comments, missed those in my review, sorry.

Latest changes LGTM 🚀

[ramonjd]

I appreciate the quick review!

[noisysocks]

Committed in r58797.