Background: add background attachment support to theme.json styles

[ramonopoly]

A ticket to track the syncing of the PHP files for following Gutenberg PR:

• ​https://github.com/WordPress/gutenberg/pull/61382

It introduces the ability to specify a value for background.backgroundAttachment in theme.json styles, which sets a CSS value for the background-attachment property.

Syncing changes from ​https://github.com/WordPress/gutenberg/pull/61382

[!NOTE]
This PR should be committed after ​https://github.com/WordPress/wordpress-develop/pull/6836 to ensure a clean rebase 😄

## Testing

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

To test, define a background image for the site in theme.json.

Here's an example using an image in src/wp-content/themes/twentytwentyfour/

{
        "$schema": "../../schemas/json/theme.json",
        "version": 3,
        "settings": {
                "appearanceTools": true
        },
        "styles": {
                "background": {
                                "backgroundImage": {
                                        "url": "file:./assets/images/art-gallery.webp"
                                },
                                "backgroundAttachment": "fixed"
                }
        }
}

Now check the site:

​https://github.com/user-attachments/assets/6679d670-b306-45b4-858a-0b490bee5595

Trac ticket: https://core.trac.wordpress.org/ticket/61720

I've rebased on top of ​https://github.com/WordPress/wordpress-develop/pull/6836 and have added theme json block-level tests.

One thing to note is that ​https://github.com/WordPress/wordpress-develop/pull/6836 opens up block-level global styles, so I've updated the theme.json test snippet in the description.

🍺

@ramonjd: Is this ready to commit?

Is this ready to commit?

Oh yes, please!!

I intended that as a nudge for you... 😂 You win this round, I was unclear.

I intended that as a nudge for you... 😂 You win this round, I was unclear.

Haha. No worries. I mistakenly assumed there was some rule about committing your own patches.

[ramonopoly]

In 58834:

Background: add background attachment support to theme.json styles

Introduces the ability to specify a value for background.backgroundAttachment in theme.json styles.

The theme.json value determines the CSS value for the background-attachment property.

This feature was introduced into the Gutenberg plugin in version 18.9.

Props andrewserong, mukesh27, noisysocks, ramonopoly.

Fixes #61720

Merged in [58834]

​https://github.com/WordPress/wordpress-develop/commit/c3e1d3a8b8c2393ce8d6d50cededbea479516b8e

Thanks!

[kebbet]

@ramonopoly Please milestone this ticket in the 6.7 milestone, since changesets where added during this cycle. Thanks!

[ramonopoly]

Thanks for the nudge @kebbet

Done