Get the direction of the focus change for the given event.
For example, in a VisualElementFocusRing, when the Tab key is pressed, the focus change
direction should be VisualElementFocusChangeDirection.right.
When the EventDispatcher processes navigation and pointer events, it automatically calls
this method to generate Focus events.
The Focus event's IFocusEvent.direction matches the value returned by this method to
generate it.