Backport aspect ratio block support

[andrewserong]

Backport the aspect ratio block support feature from the Gutenberg plugin, that originally landed in: WordPress/gutenberg#56897 and WordPress/gutenberg#58414

Note this PR backports the PHP parts of the block support. This can be safely done prior to the JS packages landing for core, as the UI for the feature will not be exposed until the JS packages are updated.

• In a post or page add the following markup that contains a Cover block with an aspect-ratio set:

<!-- wp:cover {"customOverlayColor":"#bd6c6c","isUserOverlayColor":true,"isDark":false,"style":{"dimensions":{"aspectRatio":"16/9"}},"layout":{"type":"constrained"}} -->
<div class="wp-block-cover is-light"><span aria-hidden="true" class="wp-block-cover__background has-background-dim-100 has-background-dim" style="background-color:#bd6c6c"></span><div class="wp-block-cover__inner-container"><!-- wp:paragraph {"align":"center","placeholder":"Write title…","fontSize":"large"} -->
<p class="has-text-align-center has-large-font-size">A cover block in 16:9</p>
<!-- /wp:paragraph --></div></div>
<!-- /wp:cover -->

• Note: at this stage you won't yet see the aspect ratio rendered in the editor — this is to be expected as JS rendering will only happen once the JS packages are updated.
• Save the post or page and load the post on the site frontend. You should see that the aspect ratio rules are output for this block, along with a min-height:unset rule:

Trac ticket: https://core.trac.wordpress.org/ticket/60365

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[github-actions]

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[tellthemachines]

Thanks for the PR! Couple of minor questions below but overall LGTM!

[tellthemachines]

Would it be worth adding an @since for this change in compute_style_properties?

[andrewserong]

Good idea! Added in: 03d2d01

[tellthemachines]

Any reason for using this particular theme as opposed to, say, block-theme-child?

[andrewserong]

Yes, block-theme-child-with-fluid-typography has appearanceTools set to true so that the block support will be in use. I think we're doing the same with the background image support, too.

[tellthemachines]

Oh I see! Might be worth consolidating some of those themes at some point but this is fine for now.

[tellthemachines]

Committed in r57491.

[andrewserong]

Wonderful, thank you @tellthemachines! 🙇