@Nonnull
    default ProtocolSubscription handler(@Nonnull Consumer<? super PacketEvent> handler) {
        return handlers().consumer(handler).register();
    }
Builds and registers the Handler.

@param handler the consumer responsible for handling the event.
@return a registered {@link ProtocolSubscription} instance.
@throws NullPointerException if the handler is null
helper/src/main/java/me/lucko/helper/event/functional/protocol/ProtocolSubscriptionBuilder.java