Remove Yarn for installing packages and running scripts

Yarn 1.2.2 (also known as Yarn Classic) has been deprecated for a while. Since we're not using Yarn for anything other than running scripts, it can be easily replaced by `npm`, or `pnpm`.

Removing Yarn also prevents people from messing with the pnpm-lock.yaml file, as `pnpm` won't try and use registry.yarnpkg.com to download dependencies by mistake.