[push-api] unregister() should be available on the PushRegistration object (#94)

I think it would be great to follow the pattern of ServiceWorker:
- unregister() should live in PushRegistration;
- unregister() should return a Promise<boolean> that says whether the unregistration happened.

CC: @annevk and @jakearchibald 

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Reply to this email directly or view it on GitHub:
https://github.com/w3c/push-api/issues/94