Feature request: Rushjs monorepo support

[KevinGrandon]

What would you like Renovate to be able to do? We have recently moved to a monorepo managed by Rushjs. Rush is similar to yarn workspaces, and projects are defined by entries in a top-level rush.json file. Currently it seems that there's some manual work to update rush lockfiles, and it would be great if renovate did this out of the box.

Describe the solution you'd like Enable renovate to work with rushjs monorepos. Perhaps custom lockfile logic could occur when detecting a project with a top-level rush.json file and a common/ folder.

Describe alternatives you've considered We might be able to handle lockfile updating ourselves, with a custom CI runner that builds and updates the lockfile.

Additional context We experienced this with the Fusion.js codebase after we migrated to a monorepo. We saw that renovate was opening up Pull Requests, but not updating the common lockfile.