fix: fix es6 import file extension

tests/fetchStats.test.js

@@ -1,8 +1,8 @@
 import "@testing-library/jest-dom";
 import axios from "axios";
 import MockAdapter from "axios-mock-adapter";
-import calculateRank from "../src/calculateRank";
-import fetchStats from "../src/fetchers/stats-fetcher";
+import calculateRank from "../src/calculateRank.js";
+import fetchStats from "../src/fetchers/stats-fetcher.js";
 
 const data = {
   data: {