Give support to pnpm workspace

[smoglica]

I'm having this issue in my github action.

My repo is a monorepo handled with pnpm workspace. In the root package.json file I have some package referencies like:

{
  "devDependencies": {
    "@package-a": "workspace:*",
    "@package-b": "workspace:*",
    "@package-c": "workspace:*"
  }
}

For fixing this, probably should be enough to detect which package manager the repo is using and use the correct CLI command.

[charlespwd]

👋 I think this is fair, but I don't have the time to look at this right now (juggling with too many things).

If this can't wait, it should be rather easy to add to https://github.com/Shopify/theme-check-action/blob/main/src/steps/installCli.ts#L6-L22

A PR would be absolutely welcome!

[charlespwd]

Pushed to v2.1.0, v2.1, and v1

[charlespwd]

Had to revert #39 because of an issue with pnpm now being required if there's a pnpm lock file. Which forces folks to do setup-node with pnpm, and then there was another problem.

[charlespwd]

Following up on the comment in #39: I'm not sure I'd like a major bump for this. Ideally we want low friction and toil and asking for folks to upgrade is a lot of friction. The goal is to be almost invisible. Set it, forget it and let it do its job.

Given how we had to revert because it used to work and now no longer does. I feel like we could fallback to npm when pnpm is not available. We might be really close to have what we need.

• use detect logic to figure out what is being used by the repo
• make sure the one detected is available
  • fallback to npm when it isn't
• figure out why pnpm doesn't like --no-lockfile (or figure out the proper flag that lets you install without lockfile changes)
• consider making a global install (?)