add
Registers a `PlaylistListener` to receive notifications about playlist events.
Once added, the listener will be invoked when events such as track changes, playlist modifications, or playback state updates occur.
Parameters
listener
The `PlaylistListener` to be added. Must not be null. If a null listener is provided, the behavior is undefined and may result in a `NullPointerException` at a later time. It is the caller's responsibility to ensure the listener is valid. Adding the same listener multiple times will result in it being notified multiple times for each event.