The package manager used has been upgraded from Yarn v1 to Yarn v3. All
of the steps in the migration guide [1] have been followed. Effectively
everything should work the same way it did before.

Closes #847

[1]: https://yarnpkg.com/getting-started/migration

The old `setup` script has been replaced with a Yarn v3 plugin that
will automatically run `allow-scripts` after install. We can now use
`yarn install` again as normal.

This was done to address a problem that happened when running
`yarn setup` for the first time. If Yarn 3 wasn't installed already, it
would throw an error. Yarn 3 doesn't automatically install itself until
you run the `install` command directly.

The `setup` script has been left behind for now so that it still works
if people run it accidentally. We can remove it later once we've become
accustomed to this new workflow.

Co-authored-by: Elliot Winkler <elliot.winkler@gmail.com>

This file gets auto-generated by Yarn, so there is no need for us to
lint it.

The plugin has been reinstalled from the LavaMoat repository. Comments
have also been removed from the `.yarnrc.yml` file.