Add interaction helper for custom edges
While working on !189 (merged), I noticed that custom edges can be hard to select compared to regular ReactFlow edges.
After investigation, it seems that ReactFlow adds a wider invisible path on the top of regular edges to ease the selection: https://github.com/wbkd/react-flow/blob/f79620f4c0c1ab96eb01986988e50eb5274f9d54/packages/core/src/components/Edges/BaseEdge.tsx#L30.
This PR adds the same kind of path for custom edges.
attn @koumouts