REST API Debug/Dev Mode

[TimothyBlynJacobs]

I think we should consider adding some additional debug/dev helpers for the REST API when WP_DEBUG is active. When in this mode we'd provide additional validation that the developer is following the REST API paradigms correctly, validation that would be too intrusive or slow in production.

Some things that come to mind.

1. Verify that a developer is not calling die() in a response handler instead of using a WP_REST_Response object. This would be on top of #36271. I still see this in the wild.
2. Validating that the schemas themselves are valid.