Add support for `pnpm`

[MichaelDeBoey]

Currently, this Action is only looking for an package-lock.json/npm-shrinkwrap.json or yarn.lock. It would be awesome if it would look for pnpm-lock.yaml/pnpm-lock.yml as well.

https://github.com/bahmutov/npm-install/blob/04d753f67d29cb02b7f29f99eea1c35d70053ba3/index.js#L148

Use-case for this: we're using this Action in all @remix-run default stacks (indie-stack, blues-stack & grunge-stack), but people using Remix are either using npm, pnpm or yarn.