feat: adding Google Tag Manager to web (#262)

web/index.html

@@ -56,6 +56,13 @@
 
   <title>I/O Pinball Machine - Flutter</title>
   <link rel="manifest" href="manifest.json">
+  <script async src="https://www.googletagmanager.com/gtag/js?id=UA-67589403-1"></script>
+  <script>
+    window.dataLayer = window.dataLayer || [];
+    function gtag(){dataLayer.push(arguments);}
+    gtag('js', new Date());
+    gtag('config', 'UA-67589403-1');
+  </script>
   <script src="/__/firebase/8.9.1/firebase-app.js"></script>
   <script src="/__/firebase/8.9.1/firebase-firestore.js"></script>
   <script src="/__/firebase/init.js"></script>