How to align the non-positioned and partially-positioned children in the stack.
The non-positioned children are placed relative to each other such that the points determined by alignment are co-located. For example, if the alignment is Alignment.topLeft, then the top left corner of each non-positioned child will be located at the same global coordinate.
Partially-positioned children, those that do not specify an alignment in a
particular axis (e.g. that have neither
bottom set), use the
alignment to determine how they should be positioned in that
Defaults to AlignmentDirectional.topStart.
final AlignmentGeometry alignment