From the 2D scrolling proposal: [flutter.dev/go/2D-Foundation](https://flutter.dev/go/2D-Foundation) ✅  updated 4/25

Fixes https://github.com/flutter/flutter/issues/125505

Follow up issues:
- https://github.com/flutter/flutter/issues/126297
- https://github.com/flutter/flutter/issues/126298
- https://github.com/flutter/flutter/issues/126299
-  https://github.com/flutter/flutter/issues/122348

This adds a mostly abstract foundation for 2D scrolling in Flutter.

With these base classes, developers will be able to construct widgets that scroll in both dimensions and can lazily load their children for the best performance. This implementation is meant to be flexible in order to support different kinds of 2D compositions, from tables to scatter plots. 

The upcoming TableView, TreeView, etc widgets (coming soon in flutter/packages) are built on top of this foundation.