Challenges

1. Add /postcode/3021 json response
2. Add sendHTML(response, mainHTML) function to utils.js sendHTML(response, `

   About

   This is a paragraph

   `)
3. Change the not found route to send back JSON instead of plain text, while still keeping the 404 status code — e.g. { "error": "Page not found" }
4. Add /assets/main.css route with some simple CSS
5. Change sendHTML() to add a <link> to the main.css URL
6. (Advanced) Add an assets directory with an gif file inside. Serve that file at /assets/example.gif with the correct content type.
7. (Advanced) Add a catch-all /postcode/(number) route, which reads a matching file in a postcodes directory. e.g. /postcode/3040 will read the file at /postcodes/3040.json in the project directory, and send it back. Otherwise, if no matching file is found, it will 404