Introduce an iOS-only method for verifying the push notification permissions

### Problem description

There is no way to check if the push notification permissions have been enabled. It is a common use case to check if the user really did grant the permission or not in order to display a corresponding message to him and would be required for apps, which primary rely on push notifications to function properly.

(Currently, this could be done using a plugin like `cordova-plugin-diagnostics`, however it declares generic description for various iOS permissions, which might not be desired for production apps.)

### Expected behavior

* Support a method to check if the notifications permission has been enabled, similarly to https://github.com/dpa99c/cordova-diagnostic-plugin#isregisteredforremotenotifications

* Support a method to check if notification banners have been enabled, similarly to https://github.com/dpa99c/cordova-diagnostic-plugin#isremotenotificationsenabled.