What problem does this address?

The current PHP API does not have the functionality to create a notification.

What is your proposed solution?

I propose add a method to the channel class that emits a notification from the registered channel:

$channel->emit( 
  array(
    'message' => __( 'Testing, testing 1, 2, 3... testings', 'wp-feature-notifications' )
  )
);

Questions

• Does attaching it to the registered channel object make the most sense?