Expose pushManager on Window

This is part of the Declarative Web Push initiative (see #360) and mainly makes sense when that is supported, although could be independently supported in theory.

This makes window.pushManager work by making push subscriptions tied to a scope rather than a service worker registration. Most often push subscriptions remain 1:1 with service worker registrations, but the scope whose serialized path is "/" is treated specially from now on and can exist independently.

This obsoletes #368.