feat: Implement ScrollCallbacks on new event system

[luanpotter]

Description

Following up on our events master plan.

This adds ScrollCallbacks and necessary infrastructure to make it work.

This replaces the old ScrollDetector (which we can deprecate later), e.g.:

class MyGame extends FlameGame with ScrollDetector {
  @override
  void onScroll(PointerScrollInfo info) {
    camera.moveBy(-info.scrollDelta.global);
  }
}

With ScrollCallbacks following the newer patterns in flame:

class ScrollableSquare extends RectangleComponent with ScrollCallbacks {
  @override
  void onScroll(ScrollEvent event) {
    final factor = event.scrollDelta.y > 0 ? 1.1 : 0.9;
    scale.scale(factor);
    event.continuePropagation = false;
  }
}

Also updates the example

Checklist

• I have followed the Contributor Guide when preparing my PR.
• I have updated/added tests for ALL new/updated/fixed functionality.
• I have updated/added relevant documentation in docs and added dartdoc comments with ///.
• I have updated/added relevant examples in examples or docs.

Breaking Change?

• Yes, this PR is a breaking change.
• No, this PR is not a breaking change.

[spydon]

Lgtm, just some minor doc comments