Add pnpm audit support

It would be great if salus supported pnpm support in addition to npm and yarn. The output of `pnpm audit --json` is identical to NPM, so hopefully this is a simple request :)

One caveat to be aware of is that pnpm places the lockfile at the the root of the repository in a monorepo/workspaces setup. For example:

```
package-a/
package.json
package-b/
package.json
package.json
pnpm-lock.yaml
```

If `package-a` had its own `salus-config.yaml`, and we were running the scanner with `--repo_path=/some/path/to/package-a`, the `should_run` check would need to recursively look upwards for the folder containing `.git` and see if `pnpm-lock.yaml` exists at the same level.