Whether to place first child at the start of the container or the last child at the end of the container, when the viewport has not been offset.
For example, if the mainAxis is Axis.vertical and
there are enough items to overflow the container, then
ViewportAnchor.start means that the top of the first item
should be aligned with the top of the viewport with the last
item below the bottom, and ViewportAnchor.end means the bottom
of the last item should be aligned with the bottom of the
viewport, with the first item above the top.
This also affects whether, when an item is added or removed, the displacement will be towards the first item or the last item. Continuing the earlier example, if a new item is inserted in the middle of the list, in the ViewportAnchor.start case the items after it (with greater indices, down to the item with the highest index) will be pushed down, while in the ViewportAnchor.end case the items before it (with lower indices, up to the item with the index 0) will be pushed up.