The Axis along which the scroll view's offset increases.
For the direction in which active scrolling may be occurring, see ScrollDirection.
Defaults to Axis.vertical.
This property only applies to the Axis of the outer scroll view, composed of the slivers returned from headerSliverBuilder. Since the inner scroll view is not directly configured by the NestedScrollView, for the axes to match, configure the scroll view of the body the same way if they are expected to scroll in the same orientation. This allows for flexible configurations of the NestedScrollView.
final Axis scrollDirection;