Create IDragger and IDraggable interfaces

[BeksOmega]

interface IDragger {
  constructor(dragged: IDraggable);

  // Lets the dragger handle any drag startup.
  onDragStart(e: PointerEvent);

  // Lets the dragger calculate where the element should actually be moved to.
  onDrag(e: PointerEvent, totalDelta: Coordinate);

  // Lets the dragger handle any drag cleanup.
  onDragEnd(e: PointerEvent);
}

interface IDraggable {
  // Returns true if the element is currently movable. False otherwise.
  isMovable(e: PointerEvent): boolean;

  // Returns the current location of the draggable in workspace coordiantes.
  getLocation(): Coordinate;

  // Lets the element handle any drag startup (e.g. moving to the front of the workspace)
  startDrag(e: PointerEvent);

  // Lets the element handle moving the new location, and updating any visuals based
  // on that (e.g. connection previews).
  drag(newLoc: Coordinate, target: IDragTarget, e?: PointerEvent);

  // Lets the element handle any drag cleanup.
  endDrag(e: PointerEvent);
}

See PR #7846 for initial implementation.