Fix 2D tap to stop scrolling (#138442)

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