Clean up scrollable.dart for 2D

[Piinks]

Part of the 2D scrolling proposal: flutter.dev/go/2D-Foundation

Part of #122355 (tests will be migrated separately for validation)

This relocates some public APIs to another file in order to make some breathing room for TwoDimensionalScrollable.

• ScrollableDetails
• EdgeDraggingAutoScroller
• ScrollIncrementCalculator
• ScrollIncrementType
• ScrollIncrementDetails
• ScrollIncrementIntent
• ScrollAction

I also tidied up some getters that were scattered throughout ScrollableState.

This required exposing 2 new getters on ScrollableState

• resolvedPhysics
  • I actually think this is super beneficial to add. The way physics are applied internally can be difficult for users to debug. This makes what the actual physics ended up being way more accessible.
• deltaToScrollOrigin
  • Used by the EdgeDraggingAutoScroller. Simple look up that seems harmless to expose, it is a simple method for computing the axis oriented absolute value of the distance from the 0 position.

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I read and followed the Flutter Style Guide, including Features we expect every widget to implement.
• I signed the CLA.
• I listed at least one issue that this PR fixes in the description above.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or this PR is test-exempt.
• All existing and new tests are passing.

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[reidbaker]

I have a mild preference for scrollable_helpers to have its own test file but I can see the value in showing that the existing tests still pass after the move.

[Piinks]

I concur! I will move the related tests into a new file in a separate PR to ensure ensure everything still checks out. 👍