fix ESM build

Author: endel

package.json

@@ -1,6 +1,6 @@
 {
   "name": "@colyseus/playground",
-  "version": "0.16.1",
+  "version": "0.16.2",
   "main": "./build/index.js",
   "module": "./build/index.mjs",
   "typings": "./build/index.d.ts",

src-backend/index.ts

@@ -3,6 +3,7 @@ import express, { Router } from 'express';
 import { auth, JWT } from '@colyseus/auth';
 import { matchMaker, IRoomCache } from '@colyseus/core';
 import { allRoomNames, applyMonkeyPatch } from './colyseus.ext.js';
+import { fileURLToPath } from 'url'; // required for ESM build (see build.mjs)
 
 export type AuthConfig = {
   oauth: string[],