Components: Assess stabilization of Theme

[fullofcaffeine]

Issue: #59418.

What?

This is part of a larger effort to remove __experimental prefix from all "experimental" components, effectively promoting them to regular stable components. See the related issue for more context.

Why?

The strategy of prefixing exports with __experimental has become deprecated after the introduction of private APIs.

How?

1. Export it from components without the __experimental prefix;
2. Keep the old __experimental export for backwards compatibility;
3. Change all imports of the old __experimental in GB and components to the one without the prefix (including in storybook stories). Also, update the docs to refer to the new unprefixed component;
4. Add the component storybook id (get it from the storybook URL) to the PREVIOUSLY_EXPERIMENTAL_COMPONENTS const array in manager-head.html so that old experimental story paths are redirected to the new one;
5. Add a changelog for the change.

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[tyxla]

@mirka @WordPress/gutenberg-components do you recall why we kept Theme only as a a private API? Seems like a miss to me, did we mean to actually export it as a public API as well?

It also appears like we still claim the component to be "experimental" in the README:

gutenberg/packages/components/src/theme/README.md

Line 4 in dc8b7c8

This feature is still experimental. “Experimental” means this is an early implementation subject to drastic and breaking changes.

[ciampo]

do you recall why we kept Theme only as a a private API

If I remember correctly, it was discussed that the private Theme component in @wordpress/components would be ultimately removed in favor of a separate @wordpress/theme package (see for example #56669), so that we could create theming APIs that are more comprehensive and can power all of Gutenberg's UI (instead of just the components package).

[tyxla]

That makes sense, but then I fail to understand the purpose of this PR. I guess @mirka might know.

[mirka]

The purpose of this PR was simply to normalize the URL/grouping of the Theme component in Storybook. It's still private and will likely never be public as @ciampo says.