Wait for <x> and <y>

The `wait` syntax in hyperscript does a great job of enabling event controlled flow. However, it has trouble when waiting on more than one event at the same time. For example:

```
on closeModal(event)
add .closing to #modal
async do
if theURL is not null then
fetch theURL then
put it into #main.innerHTML
end
trigger fetchDone
end
wait for animationend
wait for fetchDone
remove #modal
end
```

This should work if `animationend` finishes before `fetchDone`, but it deadlocks if `fetchDone` completes first.

I think hyperscript would benefit from a `wait for x and y` syntax that is similar to `Promise.all()` that only returns after all of the events have triggered.

This might be generalized to other kinds of operations, such as `wait for any of x and y and z` but my first guess is that this would be less commonly useful.

Comienza localizando la sintaxis `wait` de hyperscript y el flujo controlado por eventos descrito en el issue. Usa el ejemplo proporcionado con `animationend` y `fetchDone` para examinar el comportamiento cuando cualquiera de los dos eventos ocurre primero. Se considera completado cuando una forma `wait for x and y` reanuda la ejecución solo después de que ambos eventos se hayan disparado, sin bloquearse cuando cambia su orden.

Escrito por el modelo de indexación a partir del texto del issue.