refactor app, move to src folder and separate api folder

src/index.js

@@ -0,0 +1,16 @@
+const express = require('express');
+const helmet = require('helmet');
+const app = express();
+const PORT = process.env.PORT || 3000;
+
+const api = require('./api');
+
+app.use(helmet({
+    crossOriginResourcePolicy: false,
+  }));
+
+app.use('/', api.router);
+
+app.listen(PORT, () => {
+  console.log(`🚀 Badge API running at http://localhost:${PORT}/`);
+});