use PositionRetainedScrollPhysics

[SimonWang9610]

Problem:

• using RetainableScrollController may break developers' existing code implementations
• and developers have to invoke RetaibleScrollController.retainOffset() explicitly to achieve their goals.

Solution:
By deprecating RetainableScrollController, we could use PositionRetainedScrollPhysics to achieve the same goals, but without introducing breakage into your existing projects. The usage is like below:

ListView.builder(
  controller: _controller,
  reverse: true,
  physics: const PositionRetainedScrollPhysics(),
  itemBuilder: (context, index) => _items[index],
  itemCount: _itemCount,
);

The only thing would change is passing physics: const PositionRetainedScrollPhysics() in your ListView. Particularly, you could also disable retaining the scroll position by setting PositionRetainedScrollPhysics.shouldRetain = false. Besides, it could also be applyTo other ScrollPhysics.