[view-transitions] Expose `activeViewTransition`

[bramus]

The spec refers to the active view transition which is added to the Document. However, this is not actually exposed to authors.

The React folks have asked me about this before in some conversations, and they are working around it by manually keeping track of the active view transition whenever startViewTransition is called.

I think it would make sense to add Document.activeViewTransition (and also Element.activeViewTransition for scoped transitions) for easier access.