Support for wrapping the resulting widget in AnimationConfiguration.toStaggeredList() in another widget, also in an Expanded widget

[souradeep98]

This adds a new optional parameter wrapperBuilder in the helper function AnimationConfiguration.toStaggeredList().

This function can be used to wrap the resulting widget of a child widget, located at any index in the children list, in another widget.

Declaration
Widget? Function(int index, Widget child)? wrapperBuilder

Motivation
This helper function wraps each of the children in a AnimationConfiguration.staggeredList. So, you cannot wrap your widget in a Expanded or Flexible widget, as they needs to be directly placed under a Flex widget like Row or Column. This function gives the opportunity to do that, by letting you wrap the resulting widget wrapped in AnimationConfiguration.staggeredList, in another widget.

Use Cases:
This function can be used to wrap your widget in Expanded or Flex widget.

How to use?
If you want to wrap the resulting child widget at index 2 in your children list in an Expanded widget, you can do that using this function like this:

wrapperBuilder: (index, child) {
  if (index == 2) {
    return Expanded(
      child: child,
    );
  }
  return null;
},

if null is returned, the resulting widget wrapped in AnimationConfiguration.staggeredList is used.

The parameter is optional. If not passed, the resulting widgets wrapped in AnimationConfiguration.staggeredList are used.