Can we use Webpack and TypeScript in our Drupal themes?

Yes. We. Can. Here's a Drupal theme with Webpack configured to transpile sass/scss to css and TypeScript to JavaScript: https://github.com/codingsasi/nephos

1. Download this theme into your themes/custom directory. This theme is named "nephos", after the greek goddess of the internet. But you can use the bash script to change the name quite easily.
2. Run npm install.
3. Start working on your theme. The SCSS files are in the scss folder and TypeScript files are in the js folder. You can write code in JS as well, just remember to import it in nephos.ts file because that's the "entry".
4. Run npm run build --mode=development and the compiled files will be in this dist folder.
5. Use the bash script change-theme-name.sh to refactor the theme name.