Ease droppable orientation restrictions

[MaximSagan]

The problem

As documented in #1906, a draggable currently cannot be moved by keyboard to another droppable if the droppables are oriented in the same direction as the draggables).

Internally, this was related to the behavior in src/state/move-in-direction/index.js where moveToNextPlace() (move internally) would be called if the arrow key is in the same axis as the draggable's direction (isMovingOnMainAxis === true), and moveCrossAxis() (which allows movement between droppables) would be called if the arrow key is on the other axis (isMovingOnMainAxis === false).

The problem here (and it is clear from the function name moveCrossAxis) is that it pre-supposes that droppables must be oriented along the cross axis to the main axis that the draggables are oriented along. This is overly restrictive, especially when we consider vertical lists where the number of draggables and droppables are both dynamic and it is unrealistic to place droppables horizontally side-by-side without undesirable horizontal scrolling.

What I've done

I have naively renamed moveCrossAxis to moveAxis (this involved renaming a directory). It is now also being called when isMovingOnMainAxis === true if moveToNextPlace yielded a null result (i.e. there was no internal position that the draggable could be moved to, which should indicate that the first/last position within the droppable has been reached and the user is trying to reach another droppable).

I have added the isMovingOnMainAxis boolean as an option for moveAxis, and it is being passed to getBestAxisDroppable (formerly getBestCrossAxisDroppable) where the majority of the changes have been made (to support both axes).

As this represents a change in functionality (purely additive in my opinion), I have updated the docs as well.

Note

I have not added any unit tests for this for this new functionality. As I do not expect this PR to be approved and merged in its current form, I thought I should wait before adding any so as not to waste time.

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