Re: [push-api] Use ExtendableEventInit instead of EventInit (#78)

Also note that when an attribute is added to `ExtendableEvent` and its initializer into `ExtendableEventInit`, the attribute won't be initialized when `PushEventInit` directly inherit from `EventInit`.

At least, we should do either:
- Make PushEventInit inherit from ExtendableEventInit
- Make PushEvent directly inherit from EventInit (not ExtendableEventInit)


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