[intersect] enhance with more events & scroll direction detection

[vnphanquang]

Context

It's more convenient to have an on:intersectleave & on:intersectenter events, especially in use cases to track if element is in view.

Also it's helpful sometimes to know the scrolling direction.

Inspiration might be pooled from https://github.com/maciekgrzybek/svelte-inview/blob/master/src/inview.ts

Todos

• support both axes in direction detection (currently only y)
• add intersectenter & intersectleave events