feat: use drag direction enum and helpers

[rachel-fenichel]

Follow-up on #382.

• Pass a Direction to constrained and unconstrained drags.
• Unpack xy and use to calculate drag delta for unconstrained drags.
• Pack xy into a fake pointer event for constrained drags.
• Unpack fake pointer information in keyboard_drag_strategy and add isConstrainedMovement to minimize contact with the fake pointer event attributes.