Update is_fse_active to utilize FSE helper function

[noahtallen]

This past week, we added a helper function to FSE which tells us when FSE is active on a site (which has now been deployed). I'm currently updating our API work to use this function, as it centralizes most of the logic needed! (Including all of the previous checks in this function.) Now we'll be able to update anything necessary without having to modify a bunch of API endpoints all the time. (yay!)

This is a bit of a followup to #13392. Testing instructions are practically the same as in that PR since behavior should be identical.

Changes proposed in this Pull Request:

• Uses \A8C\FSE\is_full_site_editing_active(); to replace logic in is_fse_active endpoint.
• From the user's pov, there is no functional change with this PR. It simply improves code quality :)

Testing instructions:

1. Create a jurassic ninja site with this branch: https://jurassic.ninja/create/?branch=update/use-fse-active-helper-function-in-plugin. (also connect it to jetpack so you can see it in Calypso)
2. Before doing anything, use the api console to make sure that is_fse_active is false without the plugin installed. We're using the endpoint /sites/abc.jurassic.ninja
3. Install the Full Site Editing Plugin, Gutenberg development plugin and activate both
4. Check to make sure that is_fse_active is still false
5. Using calypso (URL like https://wordpress.com/themes/abc.jurassic.ninja), activate the "Maywood" theme
6. Now, in the console, is_fse_active should be true.
7. Verify that you can see the FSE editor by editing a page.
8. Add the following filter to the site, can be done easily in the plugin editor: add_filter( 'a8c_disable_full_site_editing', '__return_true' );
9. Now, in the console, is_fse_active should be false.
10. FSE flows for the post editor should not be available, but other features still are like Starter Page Templates (which you'll see creating a new page) and Recent Blog Posts

[jetpackbot]

	Warnings
⚠️	The PR is missing at least one [Status] label. Suggestions: [Status] In Progress, [Status] Needs Review
⚠️	"Proposed changelog entry" is missing for this PR. Please include any meaningful changes

This is an automated check which relies on PULL_REQUEST_TEMPLATE. We encourage you to follow that template as it helps Jetpack maintainers do their job. If you think 'Testing instructions' or 'Proposed changelog entry' are not needed for your PR - please explain why you think so. Thanks for cooperation 🤖

Generated by 🚫 dangerJS against 3588eeb

[jeherve]

This works well in my tests. 👍

[gwwar]

The Gutenberg plugin is also required to be installed for full site editing flows to work properly :D

[gwwar]

Thanks @noahtallen this tests well for me, and the site response now also matches in /me/sites vs /sites/site with D32626-code applied.