#7512550 Style selection in design picker: Implement style variation in the full-screen theme preview

Proposed Changes

NOTE: This PR depends on D86697-code

This PR set the groundwork for the new full-screen theme preview, which can be referenced here. In the spirit of keeping this PR reasonably scoped, we are only implementing the “sidebar” of the component, which contains the theme title, description, style variations, support links, and theme features.

While the component “sidebar” is mostly complete, there are some work left that will be addressed in follow-up PRs:

• Style variation preview font rendering.
• Selecting style variation from the design picker should carry over to the full-screen theme preview.

This PR main does 3 things:

1. Creates a new package design-preview, which is the full-screen theme preview users will see when they click on a theme from the theme grid. The relevant files to the package are:
   

2. Retrieves theme details from the /v2/starter-designs/${slug} endpoint (diff: D86697-code). The relevant files are:
   

3. Refactor the style variation badges implemented in #66856 now that we have a better understanding of the variation data structure. The relevant files are:
   

Testing Instructions

• Head to the unified design picker /setup/designSetup?siteSlug=${site_slug}
• Ensure that clicking any generated design should display the old full-screen theme preview.
• Ensure that clicking any static design should display the new full-screen theme preview.
• Ensure that themes with style variation indicators should also display style variation iframe previews in the new full-screen theme preview.

Pre-merge Checklist

• Have you written new tests for your changes?
• Have you tested the feature in Simple (P9HQHe-k8-p2), Atomic (P9HQHe-jW-p2), and self-hosted Jetpack sites (PCYsg-g6b-p2)?
• Have you checked for TypeScript, React or other console errors?
• Have you used memoizing on expensive computations? More info in Memoizing with create-selector and Using memoizing selectors and Our Approach to Data
• Have we added the “[Status] String Freeze” label as soon as any new strings were ready for translation (p4TIVU-5Jq-p2)?

Related to #66741