Code block: Enhance core instead of new block

[sirreal]

Proposed changes:

Update the a8c/code block to enhance the core/code block instead of providing an independent block.

Known issues:

The global styles system doesn't appear to recognize the block. For some reason, it does not add styles as expected. A code block with in default twentytwentyfive has a font family, but this is never added to the inline styles when the block override is in place:

:root :where(.wp-block-code){
  font-family: var(--wp--preset--font-family--fira-code);
}

Custom block fontFamily is currently disabled. Due to this global styles issue and other complexities, fontFamily support is not available but should be added at some point.

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)?

Jetpack product discussion

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

No.

Testing instructions:

The block is not enabled and will have thorough testing before release.

Testing setup instructions are here. It's available on sites with the gutenberg-edge flag for testing. The most important thing to verify is that the feature is not enabled by default. That logic has not changed in this PR.

[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 WoA dev 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 (Jetpack or WordPress.com Site Helper), and enable the code-block/enhance-core-code branch.
• To test on Simple, run the following command on your sandbox:

bin/jetpack-downloader test jetpack code-block/enhance-core-code

bin/jetpack-downloader test jetpack-mu-wpcom-plugin code-block/enhance-core-code

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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• ✅ Add a "[Status]" label (In Progress, Needs Review, ...).
  
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This comment will be updated as you work on your PR and make changes. If you think that some of those checks are not needed for your PR, please explain why you think so. Thanks for cooperation 🤖

---

Follow this PR Review Process:

1. Ensure all required checks appearing at the bottom of this PR are passing.
2. Make sure to test your changes on all platforms that it applies to. You're responsible for the quality of the code you ship.
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[jp-launch-control]

Code Coverage Summary

Coverage changed in 2 files.

File	Coverage	Δ%	Δ Uncovered
projects/packages/jetpack-mu-wpcom/src/features/wpcom-blocks/code/class-code-block.php	0/264 (0.00%)	0.00%	75 💔
projects/packages/jetpack-mu-wpcom/src/features/wpcom-blocks/code/class-code-block-html-replacer.php	0/62 (0.00%)	0.00%	8 💔

Full summary · PHP report · JS report

_{Coverage check overridden by I don't care about code coverage for this PR Use this label to ignore the check for insufficient code coveage. .}

[sirreal]

The failing PHP tests are pre-existing and not caused by this PR.