[push-api] Rename hasPermission (#136)

The promise for `hasPermission` is resolved with an enum, not a boolean. This seems slightly inconsistent,  `hasPermission` suggests a boolean.

I suggest we rename it to `getPermission`.

@annevk suggested renaming it to `permission` as that's what the Notifications API facility is called. But in the Notifications API it is a property, not a method, and method names should really be verbs. Anne, what do you think?

CC @beverloo

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Reply to this email directly or view it on GitHub:
https://github.com/w3c/push-api/issues/136