Add packageManager fields for Corepack compat

[merrywhether]

Summary

For users with Yarn defaulted to >1 / Berry, setting up the repo will result in the creation of the .yarn/ directory, yarn.lock updates, and package.json edits. This change enables corepack users to automatically detect and use Yarn Classic with no effect on anyone else.

I targeted the latest version of Yarn Classic as a guess, since there were no engines entries to key off of. Happy to change this as desired.

How did you test this change?

Before this, running yarn in a fresh clone in either the root or compiler workspaces resulted in Yarn Berry making the above changes to the repo: 2000+ .yarn/cache entries, lockfile regeneration, and package.json formatting changes.

After this, yarn is handled by Yarn Classic as expected and no changes occur (and repo functions as expected).