Support for package grouping

[netroy]

Sometimes monorepos can have dozens (if not hundreds) of packages. This makes filtering very cumbersome.
For example if we want to run the tests for all the frontend packages, and there are a dozen such packages, the pnpm command ends up with a dozen --filter args (unless the repo is strict about using consistent prefixes/suffixes).
If we add a pnpm.groups property array in each package.json, we could instead do pnpm --filter-group=frontend build without having to keep adding new package names into the frontend build command.

If this is something that interests people, I'd be more than happy to try to create a PR.