This is motivated by part of the 2D scrolling proposal: [flutter.dev/go/2D-Foundation](https://flutter.dev/go/2D-Foundation)

This is one of the last little PRs to prep for the 2D scrolling foundation. 
This adds an optional `axis` parameter to the static Scrollable methods `[of, maybeOf, recommendDeferredLoadingForContext]`. This allows developers that are nesting scrollables (or one day using 2D scrolling) to look them up instead by a particular axis.

In general, even outside the context of 2D, I think this is helpful. I am often asked how to get the outer scrollable when nesting. Now it can be done.

There is also a small semantic refactor here in ScrollableState.build, this just creates a private method (_buildChrome) that will be overridden in 2D later. It is easier to add now than in the really big PR that will be.