Feature request: 'tns device --available-devices' to list all emulator images

[KristianDD]

Tell us about the problem

Now tns device android lists only android devices and tns device lists all devices. Following the logic we already have tns device android --available-devices which lists all available android emulator images, there should be tns device --available-devices to list all available android emulator images and iOS simulator images(on Mac).

Additional benefit of this will be the better visibility of --available-devices option, because currently you should call tns device ios --help to find it.