Adopted from https://github.com/flutter/flutter/pull/133750
That PR was abandoned. This finishes it up so we can land it.
Fixes https://github.com/flutter/flutter/issues/133529

Moves the `PanGestureRecognizer` used to drag the content along both axis to the outer vertical `Scrollable` subclass instead of the inner horizontal `Scrollable` subclass.

- This solves the issue of the inner `Scrollable` gestures being disabled while the outer `Scrollable` is scrolling
- Enables the user to stop the scroll movement by dragging the content again

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.