dillon1000/react

Handle fabric root level fragment with compareDocumentPosition (#34533)

The root instance doesn't have a canonical property so we were not
returning a public instance that we can call compareDocumentPosition on
when a Fragment had no other host parent in Fabric. In this case we need
to get the ReactNativeElement from the ReactNativeDocument.

I've also added test coverage for this case in DOM for consistency,
though it was already working there because we use DOM elements as root.
This same test will be copied to RN using Fantom.