fixes: #6975

• adds getAdjacentHandleForShiftSnapping() method to ShapeUtil to allow custom shapes to define which handle should be used as the reference point when dragging handles with shift key for angle snapping.

• fixes janky snapping behavior in shapes like bezier curves where control points need to snap relative to specific anchor points instead of the default "next adjacent handle" logic.

Change type

• feature

Test plan

1. Added unit tests for custom adjacent handle snapping behavior
2. All existing tests pass, ensuring backward compatibility

• Unit tests
• End to end tests

Release notes

• Added support for custom relative snap points for handles via getAdjacentHandleForShiftSnapping and snapReferenceHandleId.