feat: add simple Express app with /hello endpoint and tests

src/server.js

@@ -0,0 +1,17 @@
+const express = require('express');
+const { getGreeting } = require('./greeting');
+
+const app = express();
+const PORT = process.env.PORT || 3000;
+
+app.get('/hello', (req, res) => {
+  res.send(getGreeting());
+});
+
+if (require.main === module) {
+  app.listen(PORT, () => {
+    console.log(`Server listening on port ${PORT}`);
+  });
+}
+
+module.exports = app;