Support for drop indicator

[philippkuehn]

I would like to build something like this (it's from notion)

But it seems that it's not possible at the moment with this library, right? I figured out two problems here:

1. Support for a drop indicator. Maybe there could be a class for all .smooth-dnd-draggable-wrapper items like .smooth-dnd-drop-before and .smooth-dnd-drop-after.

2. Support for leaving the dragged item on it's original position utils it's dropped anywhere