[w3c/push-api] PushPermissionState and PermissionState are the same (#333)

The `PushPermissionState` and the Permissions API's [`PermissionState`](https://w3c.github.io/permissions/#enumdef-permissionstate) enums are the same. We should use the Permissions API one. 

Also, we should see if we can rely on the Permissions API to handle the permission prompting. 

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