Components: Expose Theme via private APIs

[tyxla]

What?

This exposes the existing Theme component via the @wordpress/component private APIs.

Why?

It's necessary for #53032, for usage outside of the @wordpress/components package.

Any prior usages of that component were within the package, so it could be directly imported.

Usage of Theme was suggested by @mirka here: #53030 (comment)

How?

• We're adding Theme to the list of @wordpress/components private APIs,
• We're removing the original README example since it was incorrect (it was assuming __experimentalTheme exposition, which isn't the case),
• We're improving the inline example.

Testing Instructions

• Verify that Theme still works well in storybook.
• Verify all checks are green.

Testing Instructions for Keyboard

None

Screenshots or screencast

None

[github-actions]

Flaky tests detected in 9ff62c0.
Some tests passed with failed attempts. The failures may not be related to this commit but are still reported for visibility. See the documentation for more information.

🔍 Workflow run URL: https://github.com/WordPress/gutenberg/actions/runs/5737134101
📝 Reported issues:

• [Flaky Test] should load styles added through enqueue_block_assets #53088 in specs/editor/plugins/iframed-equeue-block-assets.test.js

[ntsekouras]

I don't think we should add the unlocking in the example. Maybe just mention it's private? Whatever we decide here, should also apply to the other private components too(not for this PR). --cc @mirka

We could also do that in a follow up.

[tyxla]

Thanks, agreed - I was a bit torn about whether to include it or not, so I've removed that part from the example in 87e9815

Happy to do follow-up updates if anyone has a further suggestion.

[ntsekouras]

Besides the comments about readme usage, the change is straight forward. I'll pre-approve. Thanks!