Follow the steps at https://yarnpkg.com/getting-started/migration to upgrade to
Yarn v3:

 1. `yarn set version berry`
 2. Edit `.yarnrc.yml` to add `nodeLinker: node-modules`.
 3. Run `yarn install` to migrate lockfile
 4. Update `.gitignore` (see comment)
 5. Commit all new/changed files

The initial motivation for this was to fix warnings about strip-ansi-cjs,
string-width-cjs and wrap-ansi-cjs that recently appeared during fresh installs.
More generally this gets us onto an actively maintained version of Yarn.