Feature Request: pnpm is not supported for `pkg-manager`

For our official CircleCI Docker Convenience Image support policy, please see [CircleCI docs](https://circleci.com/docs/convenience-images-support-policy).

This policy outlines the release, update, and deprecation policy for CircleCI Docker Convenience Images.

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**Describe the Feature Request**
Can't use `pnpm` with this cimg because it is not supported (apparently)

**Is your feature request related to a particular problem?**
No

**How will this feature request benefit CircleCI jobs using this image?**
This image will be compatible with projects using `pnpm` as their package manager.

**Describe the solution you would like to see**
`pnpm` is supported value for `pkg-manager` for this cimg, and `pnpm` is used as the package manager when that setting is used.

**Describe alternatives you have considered**
I haven't considered any alternatives.