REST API: Add activate themes endpoint

[xavier-lc]

https://core.trac.wordpress.org/ticket/63739

What?

This PR adds the /themes/activate endpoint.

Why?

In order to be able to activate themes via REST API.

How?

The API validation follows the same logic currently used on wp-admin:

https://github.com/WordPress/wordpress-develop/blob/8a3c793e08e4374b3176424a3fece9d0906f4c6c/src/wp-admin/themes.php#L20-L35

Testing Instructions

1. Checkout this branch
2. Run npm run dev
3. Open http://localhost:8888/wp-admin/themes.php
4. Check the Active theme
5. Go to http://localhost:8888/wp-admin/post-new.php?gutenberg-demo
6. On the console, run:

await wp.apiFetch( { url: `/index.php?rest_route=%2Fwp%2Fv2%2Fthemes%2Factivate`, method: 'POST', data: { stylesheet: 'twentyfifteen' } } ); 

7. Check that the response is { "message": "Theme activated successfully.", "theme": "twentyfifteen" }
8. Go back to http://localhost:8888/wp-admin/themes.php and check that the active theme is indeed twentyfifteen

Testing Instructions for Keyboard

Screenshots or screencast

Before	After

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[spacedmonkey]

There is already a PR for this - #41836

[xavier-lc]

There is already a PR for this - #41836

@spacedmonkey thanks for pointing it out, I wasn't aware of it. The activate action is done there with the PUT method, right?

Is there any chance of that PR getting merged, given that it's from 3 years ago? Thanks!

[heavyweight]

If the theme requirements are not met switch_theme can call wp_die.
Maybe its good to handle this scenario. WDYT?

[xavier-lc]

I've checked and wp_die takes care of that. It gets into the wp_is_json_request() clause

https://github.com/WordPress/wordpress-develop/blob/54a06f8972bba2443d624372270f1ad666bcc55f/src/wp-includes/functions.php#L3779

And then the API returns this: {code: "wp_die", message: $message, data: {status: 500}, additional_errors: []} where $message is the wp_die param.