We were using Yarn Classic. While this is not deprecated, Yarn is moving
in a different direction with Yarn Modern (read more
[here](https://yarnpkg.com/migration/overview)) and we should align with
that. The migration mainly requires rebuilding our lockfile. We will
still use `node_modules` for now, though in the future we may want to
adopt the new [Plug'n'Play model](https://yarnpkg.com/features/pnp).
