Add 'viewBuilder' property in StoryItem

[Kigstom]

Its possible to add 'viewBuilder' property to the StoryItem?
This could be useful, with some other minor code cleanup, for the animated widgets, custom widgets and other lazy UI setups.

Sample

class StoryItem {
  ...

  /// The page content view for backward compatibility, but optional
  final Widget? view;

  /// The page content builder for lazy loading
  final Widget Function(BuildContext context)? viewBuilder;

  StoryItem({
    required this.index,
    required this.duration,
    this.isShown = false,
    this.view,
    this.viewBuilder,
  }) : assert(view != null || viewBuilder != null, 'Must provide a `view` or a `viewBuilder`!');

  ...
}

class StoryViewState extends State<StoryView> with TickerProviderStateMixin {
  AnimationController? _animationController;
  Animation<double>? _currentAnimation;

  StreamSubscription<PlaybackState>? _playbackSubscription;

  VerticalDragInfo? verticalDragInfo;

  StoryItem? get _currentStory {
    return widget.storyItems.firstWhereOrNull((it) => !it!.isShown);
  }

  Widget get _currentView {
    var item = _currentStory;
    item ??= widget.storyItems.last;

    if (item == null) return SizedBox.shrink();

    // 1. Build the widget ONLY when it's about to be shown
    Widget generatedView = item.viewBuilder != null
        ? item.viewBuilder!(context)
        : (item.view ?? SizedBox.shrink());

    // 2. Wrap it with the Key to guarantee animations play
    return KeyedSubtree(
      key: ObjectKey(item),
      child: generatedView,
    );

    return item.view ?? SizedBox.shrink();
  }

  ...
}