Add graph.

src/data-structures/graph/GraphEdge.js

@@ -0,0 +1,12 @@
+export default class GraphEdge {
+  /**
+   * @param startVertex {GraphVertex}
+   * @param endVertex {GraphVertex}
+   * @param weight {number}
+   */
+  constructor(startVertex, endVertex, weight = 1) {
+    this.startVertex = startVertex;
+    this.endVertex = endVertex;
+    this.weight = weight;
+  }
+}