After #1115, new Node.js versions will be able to be used for
development of the frontend. This PR changes the recommended Node.js
version to 18 (the current LTS version). I have also tested with 16 and
20 and it works.

I also took the opportunity to:

- move the build script that was at the project root to the `bin/`
folder
- update the `browserslist` to target Electron 24 (which is the version
we currently build against). This results in a slightly smaller bundle
due to no longer having to transpile optional chaining.