Signature for builders used to generate custom layouts for AnimatedSwitcher.
The builder should return a widget which contains the given children, laid
out as desired. It must not return null. The builder should be able to
handle an empty list of
previousChildren, or a null
previousChildren list is an unmodifiable list, sorted with the oldest
at the beginning and the newest at the end. It does not include the
typedef AnimatedSwitcherLayoutBuilder = Widget Function(Widget? currentChild, List<Widget> previousChildren);