Display coming soon page template if it exists #4577134

This will add a coming soon category to the bottom of the starter page templates available on the new page screen.
Part of #45689

Testing instructions.

Note: currently this card is relatively difficult to test locally because when testing in your sandbox:

The list of all templates is loaded from php code in class-starter-page-templates.php#213
Because this code runs on the server, your hosts file will not be used, it will load the list of all templates from the production public-api.wordpress.com.
Therefore the new coming soon page will not be included in the results.

However it is possible to test locally like so:

• Checkout this branch
• Set up wp-env following the instructions here: PCYsg-ly5-p2
• Sandbox public-api.wordpress.com
• In another terminal tab, sync the api portion of this code change to your sandbox with yarn dev --sync
• On http://localhost:4013 go to add a new page.
• Scroll to the bottom of the new page template list and a “coming soon” template should be available
• Locally for me, actually creating a page with any template fails, I’m investigating this but still get the same error on master branch so can say it’s unrelated to this PR