@Nonnull
    default MergedSubscription<T> biHandler(@Nonnull BiConsumer<MergedSubscription<T>, ? super T> handler) {
        return handlers().biConsumer(handler).register();
    }
Builds and registers the Handler.

@param handler the bi-consumer responsible for handling the event.
@return a registered {@link MergedSubscription} instance.
@throws NullPointerException  if the handler is null
@throws IllegalStateException if no events have been bound to
helper/src/main/java/me/lucko/helper/event/functional/merged/MergedSubscriptionBuilder.java