Fix button block contrast checker

[talldan]

Note

This PR is not intended to merged until after #41139, as it'll make the button full-width. #41139 resolves issues with the button alignments, which then makes this PR safe to merge. (I'll keep it as a draft until then).

Proposed changes:

Fixes an issue with the Jetpack Button block's contrast checking.

The applyFallbackStyles / withFallbackStyles HOC has a few issues:

• It adds a wrapping <div> around the block which breaks a range of styles (see Jetpack button: fix width and alignment settings #41139)
• It doesn't check the correct element for background colors, which is why it doesn't work in trunk
• It doesn't update when the block's css changes (e.g if inline styles or a class are added).

The withFallbackStyles API in Gutenberg is very old, as so should probably be deprecated in core because of the flaws it has. #41139 aims to remove it, but that will still leave the contrast checker not working. This PR introduces a small hook to replace the applyFallbackStyles / withFallbackStyles to keep the contrast checker working.

Other information:

• Have you written new tests for your changes, if applicable?
• Have you checked the E2E test CI results, and verified that your changes do not break them?
• Have you tested your changes on WordPress.com, if applicable (if so, you'll see a generated comment below with a script to run)?

Does this pull request change what data or activity we track or use?

No

Testing instructions:

1. Add a contact form block
2. Select the button in the form
3. Change the text color to something that conflicts with the background (e.g. if there's a black background, set the text to black)
4. Observe the contrast checker should be displayed. (in trunk it doesn't show).

[github-actions]

Are you an Automattician? Please test your changes on all WordPress.com environments to help mitigate accidental explosions.

• To test on WoA, go to the Plugins menu on a WordPress.com Simple site. Click on the "Upload" button and follow the upgrade flow to be able to upload, install, and activate the Jetpack Beta plugin. Once the plugin is active, go to Jetpack > Jetpack Beta, select your plugin, and enable the fix/button-fallback-contrast-checking branch.

• To test on Simple, run the following command on your sandbox:

  bin/jetpack-downloader test jetpack fix/button-fallback-contrast-checking
  

Interested in more tips and information?

• In your local development environment, use the jetpack rsync command to sync your changes to a WoA dev blog.
• Read more about our development workflow here: PCYsg-eg0-p2
• Figure out when your changes will be shipped to customers here: PCYsg-eg5-p2

[github-actions]

Thank you for your PR!

When contributing to Jetpack, we have a few suggestions that can help us test and review your patch:

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