Feat: Add some hooks of presentation slides lifetime

[Sdju]

Sometimes I need to reset a state for a slide after entering. And for now I use a composable onSlideEnter

function onSlideEnter(cb: () => void) {
  const instance = getCurrentInstance();
  const { currentSlideNo } = useNav();

  watch(currentSlideNo, (slideNo) => {
    if (slideNo === instance.setupState.$page) {
      cb()
    }
  }), {  immediate: true };
}

And similar for leaving:

function onSlideLeave(cb: () => void) {
  const instance = getCurrentInstance();
  const { currentSlideNo } = useNav();

  watch(() => currentSlideNo.value === instance.setupState.$page, (_, wasActive => {
    if (wasActive)
      cb()
  });
}

Computed to check: is a slide active?

const instance = getCurrentInstance();
const { currentSlideNo } = useNav();

const isSlideActive = computed(() => currentSlideNo.value === instance.setupState.$page)

Maybe it should be a part of useNav/useNavState or... "useCurrentSlide"

The code above isn't well tested. For now it just works for my cases. I think they can be usefull for others too. But maybe I just haven't found a better way to do it with the current API.