[SuperEditor] - Drag and Drop node re-ordering

[matthew-carroll]

Add tools necessary for apps to implement drag-and-drop node re-ordering.

To prove the viability, create an example that uses long-press on mobile to re-arrange nodes. The original node should remain visible, but in a ghosted visual state. The node being re-ordered should appear in the overlay in a unique visual state. A separate decoration should be painted at the drop zone.

In the case of list items, and other nodes that are inherently parent nodes, the full hierarchy should move with the reorder.

Stretch goal: animate the node into overlay. Animate all the nodes that move down.

[alterhuman]

@matthew-carroll If it helps: #1129 (reply in thread)