Add the ability to limit Draggables to a single axis

[DaveShuckerow]

Part 1 of solving #16763.

This adds a new field to the Draggable API: axis. Behavior covered in the DartDoc, but setting it limits the underlying _DragAvatar's motion to the assigned axis. When axis is null, the behavior is the same as before. As a result, this won't break anything.

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[DaveShuckerow]

This got reformatted automatically by my editor, I will clean it up.

[DaveShuckerow]

@googlebot I have updated the authorship.

[HansMuller]

LGTM too