Upgrade to Yarn 3

This follows the instructions from https://yarnpkg.com/getting-started/migration

It ensures we keep using node_modules and don't use Plug 'n Play since
ReScript doesn't support PnP yet. See rescript-lang/rescript#3276

Yarn 3 provides us some better tools for working with monorepo's. For
example it allows us to reference a consistent version throughout the
workspace.

Author: Kingdutch