chore: put everything needed to build outside devDeps

- on deploy (on scalingo), the project is built. So it uses the build
scripts using concurrently, vite and all. We make sure those deps are
available anytime we use `npm install`
- now we can set up scalingo to not install devDeps while still
succeeding in installing the project. Actually, following their docs,
scalingo shouldn't install devDeps by default but it actually does. We
should be able to use an env var to force it bypassing devDeps
(Scalingo/nodejs-buildpack#89)