dx: bind VonageSmsChannel to container

[ankurk91]

Hi @driesvints ,

This change allow us to specify the channel class in Notification via() method.

use Illuminate\Notifications\VonageSmsChannel;

public function via() {
   return [VonageSmsChannel::class]
}

The old syntax will still work

public function via() {
   return ['vonage']
}

This helps us to CTRL+Click on class name and jump to actual class