Allow setting package manager via config

[OrbisK]

Is your feature request related to a problem? Please describe:

Stackblitz can autodetect pnpm, but not without an existing pnpm-lock.yaml file. For some starter templates, we don't want to add a lock file to the repository.

Describe the solution you'd like:
package.json

{
  "stackblitz": {
    "packageManager": "pnpm"
  }
}

Describe alternatives you've considered:

It is possible to add an empty pnpm-lock.yml to the repository to force StackBlitz to use pnpm and hydrate the content-addressable store. However, this does not work alongside the Renovate bot because it will always update the lockfile.

Additional context: