add support for google tag manager (#62)

pages/_app.js

@@ -1,5 +1,12 @@
 import '../src/styles/global.scss';
+import { useEffect } from 'react'
+import TagManager from 'react-gtm-module'
+
+const gtmId = 'G-7J55WH6BVM';
 
 export default function App({ Component, pageProps }) {
-  return <Component {...pageProps} />;
-}
+  useEffect(() => {
+    TagManager.initialize({ gtmId })
+  }, []);
+  return <Component {...pageProps} />
+}