Add getNeighbors method to Graph.

src/data-structures/graph/Graph.js

@@ -25,6 +25,13 @@ export default class Graph {
     return this.vertices[vertexKey];
   }
 
+  /**
+   * @param {GraphVertex} vertex
+   */
+  getNeighbors(vertex) {
+    return vertex.getNeighbors();
+  }
+
   /**
    * @param {GraphEdge} edge
    * @returns {Graph}