This PR fixes snapping for arrow shapes. Previously, the middle handle
of an arrow was marked as a vertex, causing the arrow to have to
segments (one of which would be snapped to). In this PR we make the
second handle a "virtual" handle and tweak how we display handles to
preserve the same appearance.
### Change Type
- [x] `minor` — New feature
### Test Plan
1. Drag an arrow while snapping.
### Release Notes
- [fix] arrow snapping
fc36d5b577