Support workspaces from package.json #2255

I have a project that is in a monorepo and uses yarn right now.

I'm considering migrating to pnpm for the monorepo, as well as all other projects I contribute to, as I have a small SSD as local storage (and with Windows...). However, it would be ideal if there was compatibility between yarn's workspaces and pnpm, so that we don't have to jump "all in" to it.

To this end, yarn allows packages in a monorepo to be defined as an array in package.json called "workspaces". I haven't checked if pnmp supports it, but from the absence in the docs, I'm assuming it doesn't.

Considering there's a separate file already, I guess one also has to consider what to do if both are present. I think it would be most intuitive if pnpm-workspace.yaml completely overrides package.json's workspaces if present (for backwards compatibility). There could also be a pnpm-workspace.yaml option on whether to change that to extend the package.json array instead.