Implement add vertex method on the graph class

Author: KarenOk

Graph.js

@@ -0,0 +1,8 @@
+class Graph {
+	constructor() {
+		this.adjacencyList = {};
+	}
+	addVertex(vertex) {
+		if (!this.adjacencyList[vertex]) this.adjacencyList[vertex] = [];
+	}
+}