Add typings for onDragUpdate
Add typings for onDragUpdate

While mentioned in the docs, attempting to use onDragUpdate will cause a type error. This PR introduces typings for onDragUpdate that are consistent with the docs:

`type DragUpdate = {|
  ...DragStart,
  // may not have any destination (drag to nowhere)
  destination: ?DraggableLocation,
|}`
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