Replace `get-workspaces` with `@manypkg/get-packages`

Hello 👋🏻 ,

I'm trying to use `workspaces-run` with `pnpm`, but unfortunately, get-workspaces doesn't recognize workspace settings from `pnpm-workspace.yaml`.

It's unlikely, that it will support pnpm in future, since package [get-workspaces](https://github.com/changesets/changesets/blob/main/packages/get-workspaces/src/index.ts) by `changesets` was deprecated and replaced with [@manypkg/get-packages](https://github.com/Thinkmill/manypkg/tree/master/packages/get-packages) ([See the migration PR there](https://github.com/changesets/changesets/pull/290)).

Could you please migrate to [@manypkg/get-packages](https://github.com/Thinkmill/manypkg/tree/master/packages/get-packages) as well?

Please let me know if you would accept a PR from my side.