refactor(notify): Remove if/else Boolean logic as in evaluates Boolean

Remove redundancy of returning `true` or `false` as `'Notification' in window` returns a Boolean, also avoids need for function calls each time.

I left `needsPermission` as a function to avoid public API change, but could be worth keeping consistent as just an Object prop + value.

Pushed the repeated `typeof` function comparisons into a reusable function, save some bytes.