Search: Migrate Classic Search, Instant Search, and Customberg to package

[jsnmoon]

Warning: #21690 must be resolved before merging this PR!

Changes proposed in this Pull Request:

This PR moves the following products to the Search package:

• Classic Search
• Instant Search along with its build process
• Customberg along with its build process

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

No.

Testing instructions:

Testing auto-configuration & Instant Search:

1. Activate this branch on a brand new site without a Jetpack Search subscription.
2. Make sure to build both Instant Search and Customberg via pnpm build within the Search package directory.
3. Purchase/add a Jetpack Search subscription to the site.
4. Ensure that the search module and instant search are both automatically enabled.
5. Ensure that a Jetpack Search with preconfigured filters is added to the site sidebar
6. Perform a site search. Ensure that the overlay uses the Expanded result format and that the sidebar renders the aforementioned search widget from (5).

Testing Customberg (search configuration interface)

7. Navigate to Customberg (/wp-admin/admin.php?page=jetpack-search-configure).
8. Ensure Customberg reads and saves instant search configuration settings as expected.

Testing Classic Search

9. Disable Instant Search using the Jetpack Search dashboard.
10. Add a Jetpack Search widget with configured filters to any available site sidebar.
11. Ensure that the filters from (8) render as expected.
12. Ensure that site search functionality works as expected.

Also, ensure that pnpm test passes within the Search package.

[github-actions]

Thank you for your PR!

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Jetpack plugin:

• Next scheduled release: February 1, 2022.
• Scheduled code freeze: January 25, 2022.

[matticbot]

Caution: This PR has changes that must be merged to WordPress.com
Hello jsnmoon! These changes need to be synced to WordPress.com - If you 're an a11n, please commandeer and confirm D69778-code works as expected before merging this PR. Once this PR is merged, please commit the changes to WP.com. Thank you!
This revision will be updated with each commit to this PR

[jsnmoon]

@jeherve, any chance I could trouble you for a review? This PR has been languishing for a couple of weeks now 😅

The bulk of this change relates to moving search products into the standalone package and refactoring their initialization logic.

[jeherve]

Let's merge this! 🚢 🔍

[github-actions]

Great news! One last step: head over to your WordPress.com diff, D69778-code, and commit it.
Once you've done so, come back to this PR and add a comment with your changeset ID.

Thank you!