@Nonnull
    default SingleSubscription<T> biHandler(@Nonnull BiConsumer<SingleSubscription<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 SingleSubscription} instance.
@throws NullPointerException if the handler is null
helper/src/main/java/me/lucko/helper/event/functional/single/SingleSubscriptionBuilder.java