Fork wordpress-develop to try utilizing new endpoints in Customizer

[westonruter]

I suppose this would be the ultimate verification that the new endpoints are complete, if we can replace the admin-ajax calls with REST API calls. It's probably not something that should just be swapped out, however, since these endpoints then also introduce the ability for us to lazy-load resources as required as opposed to exporting them all from PHP up-front (see #28580).