Capacitor: Add requestPermissions() for push notifications / Local Notifications

[corysmc]

I'd like to be able to check if the user has granted permission for notifications. This way we tell them to change it in their settings, etc.

Proposed methods
LocalNotifications.checkPermissions()
or
App.checkPermissions() (returns object of entire app permissions)

[jcesarmobile]

I went with LocalNotifications.areEnabled(), as it's not really a permission and the name is closer to the native one.

const areEnabled = await LocalNotifications.areEnabled();
console.log('are notifications enabled?', areEnabled.value);

[Finesse]

This method can be used to check if push notifications are enabled too. It would be really great if this fact is mentioned on https://capacitor.ionicframework.com/docs/apis/push-notifications because I've spend an hour to find a way to check push notifications permission.