Introduce the `xb_stark` theme that uses the Semi-Coupled theme engine, removes the need for taking over the Twig theme engine for the whole site

A nice bonus is that the Media Library widget is also rendered with xb_stark

Although XB stark defaults to using Semi Coupled and rendering with React, the MR includes Twig templates that ensure Media Library is still rendered with Twig. This was added to address an issue where core/drupal.dialog.ajax needs the completed HTML earlier than Semi coupled can (currently) provide it. If we really want to render the Media Library widget in React I'm sure this can be done, but it might be worth keeping as is since it's the format the Media Library UI is known to work well with, and there's no need for it to communicate with redux.

This may open options up to more easily address #3471978: Make the Media Library dialog look like the admin theme without materially affecting anything outside of the dialog (possibly use an iframe for CSS isolation) if we want XB specific styles instead of using the admin theme.

Changes were introduced upstream by #3469235: Make "page data" tab in right sidebar work that needed to be adjusted to this MR. Needs work:

1. The page data form is broken after the merge for some reason;
2. A form alter needs to be added back to hide the submit buttons on the page data form;
3. \Drupal\Tests\experience_builder\Functional\EntityFormControllerTest::assertFormResponse needs to be updated.

I fixed the problems I mentioned in #15, then reviewed all code changes, and approved the MR.

I have three questions/asks.

1. I left one small suggestion for rewording things in xb_stark.info.yml.
2. @bnjmnm, can you please see if you're happy with how I did the form alter in 0332d920? There might be a nicer way to do it.
3. On a call last week @jessebaker had the suggestion to use include and refer to the original Twig templates that way in our process_as_regular_twig template overrides. Would we like to do that as part of this issue, or in a follow-up?

• I left one small suggestion for rewording things in xb_stark.info.yml

  . Good call, done!

• @bnjmnm, can you please see if you're happy with how I did the form alter in 0332d920? There might be a nicer way to do it.

  It's good.

• On a call last week @jessebaker had the suggestion to use include and refer to the original Twig templates that way in our process_as_regular_twig template overrides. Would we like to do that as part of this issue, or in a follow-up?

  Thanks for the reminder! With that change this is an MR that reduces the total LOC by 139!

Just need a few additional codeowner signoffs.

Feedback addressed