buildWithChild method
A build method that receives an extra child parameter.
This method may be called with a child different from the parameter
passed to the constructor of SingleChildStatelessModifier.
It may also be called again with a different child, without this widget
being recreated.
Implementation
@override
Widget buildWithChild(BuildContext context, Widget? child) {
  return AnimatedPadding(
    key: modifierKey,
    padding: padding,
    duration: duration,
    child: child,
  );
}