Add function to merge subscriptions

[atheck]

function mergeSubscriptions<TProps, TModel, TMessage extends Message> (...subscriptions: (Subscription<TProps, TModel, TMessage> | undefined)[]): Subscription<TProps, TModel, TMessage> {
  return function mergedSubscription(model, props) {
    const results = subscriptions.map((sub) => sub?.(model, props));

    const commands = results.map((sub) => sub?.[0]);
    const disposers = results.map((sub) => sub?.[1]);

    return [cmd.batch(...commands), () => disposers.forEach((dispose) => dispose?.())];
  };
}

{
  subscription: mergeSubscription(baseSub, subscription, ...),
}