Signature for when a pointer that's dragging to select text has moved again.
The first argument
startDetails contains the details of the event that
initiated the dragging.
The second argument
updateDetails contains the details of the current
pointer movement. It's the same as the one passed to DragGestureRecognizer.onUpdate.
typedef DragSelectionUpdateCallback = void Function(DragStartDetails startDetails, DragUpdateDetails updateDetails);