This syncs changes in WPCOM from D36164-code and D36219-code, where an is_fse_eligible property is added to the Site object in the REST API. For Full Site Editing this allows the site to flag whether or not it's eligible for FSE (e.g. the plugin is installed and activated), even if the current theme doesn't support FSE.

Changes proposed in this Pull Request:

• Add is_fse_eligible property to the Get Site endpoint, e.g. https://public-api.wordpress.com/rest/v1.1/sites/<site_id>
• Add is_fse_eligible function that calls is_site_eligible_for_full_site_editing() within the Full Site Editing function — returning whether the site is eligible for FSE irrespective of whether the current theme supports FSE.

Is this a new feature or does it add/remove features to an existing part of Jetpack?

• This expands on the change introduced in Full Site Editing: add a is_fse_active property to the Site object in the REST API #13196 that added the is_fse_active property. This new property is_fse_eligible is slightly different, but has a similar implementation and has already been deployed to WPCOM in D36164-code and D36219-code, so now needs synced to Jetpack so that Jetpack sites can support the additional property.

Testing instructions:

Before checking out this change

• Connect your local site to WordPress.com via ngrok.io and activate Jetpack
• Call https://public-api.wordpress.com/rest/v1.1/sites/<site_id> from an authenticated environment, e.g. https://developer.wordpress.com/docs/api/console/ or using wpcom.req.get('/sites/<site_id_or_domain>').then( function( result ) { console.log( result ) } );'); from the console in calypso.localhost:3000.
• In the site response object you should see is_fse_active and no is_fse_eligible property.
• In the ngrok.io dashboard http://localhost:4040/inspect/http you should see that the POST request to /xmlrpc.php includes the string is_fse_eligible in its payload.

Apply this change

• Follow the above steps, and after running the API call, in the response object you should see is_fse_eligible: false
• Install the Full Site Editing plugin by Automattic from the plugins directory (/wp-admin/plugin-install.php), but don't activate it just yet
• Run the API call again, and you should still see is_fse_eligible: false
• Activate the Full Site Editing plugin, run the API call again, and you should see is_fse_eligible: true in the response object.

Proposed changelog entry for your changes:

• WordPress.com REST API: add flag to determine site eligibility for Full Site Editing.