Switch "single package manager" from Yarn to pnpm

[Josehower]

Since we have now switched from Yarn to pnpm, the error messages warning about package-lock.json in project should be updated:

✖ Use single package manager
  › package-lock.json file committed to Git. Remove it with:

    ‎  $ git rm --cached <filename>

    After you've removed it, you can delete the file with:

    ‎  $ rm <filename>

    The presence of this file indicates that npm was incorrectly used in addition to Yarn (eg. an    
    "npm install" command was run). In order to avoid issues with the state of the yarn.lock file, we    suggest also forcing regeneration this file with the following command:

    ‎  $ yarn --force

This error messages should trigger when both package-lock.json and yarn.lock are found, and recommend pnpm commands to fix the problem