feat(edges): Add source/target nodes to graph edges

# What's changed?

* Bind source and target nodes to graph edges for convenience

package/src/store/actions.ts

@@ -249,6 +249,8 @@ export default (state: State, getters: ComputedGetters): Actions => {
           ...state.defaultEdgeOptions,
           ...storedEdge,
         }),
+        sourceNode,
+        targetNode,
       })
 
       return res
@@ -287,6 +289,8 @@ export default (state: State, getters: ComputedGetters): Actions => {
         state.edges.push({
           ...state.defaultEdgeOptions,
           ...edge,
+          sourceNode,
+          targetNode,
         })
       }
     })

package/src/types/edge.ts

@@ -97,6 +97,8 @@ export interface EdgePositions {
 export type GraphEdge<Data = ElementData> = Edge<Data> & {
   selected?: boolean
   z?: number
+  sourceNode: GraphNode
+  targetNode: GraphNode
 } & EdgePositions
 
 /** these props are passed to edge components */