Replace Yarn 1.x

Yarn 1.x, our JavaScript package manager, is no longer being actively developed and is in maintenance mode, receiving only security fixes.

• https://github.com/yarnpkg/yarn/issues/8583
• https://dev.to/arcanis/introducing-yarn-2-4eh1

Before this goes EOL, we'll want to upgrade to the current version: https://yarnpkg.com/migration/guide. We could also consider moving to npm or pnpm.

Resources:

• https://github.com/microsoft/vscode/issues/162803
• pnpm demo/discussion: !4310 (closed)
• gitlab-org/frontend/rfcs#53