The public/index.html file contains a <script src='bundle.js'> tag, which means we need to create public/bundle.js. The rollup.config.js file tells Rollup how to create this bundle, starting with src/main.js and including all its dependencies, including date-fns.
npm run build builds the application to public/bundle.js, along with a sourcemap file for debugging.
npm start launches a server, using serve. Navigate to localhost:3000.
npm run watch will continually rebuild the application as your source files change.
npm run dev will run npm start and npm run watch in parallel.
MIT.