Support pnpm

[imjared]

Describe Request:

pnpm seems to be chugging along nicely and found its footing as a stable package manager in the node/js ecosystem. It's especially helpful in monorepo setups. It'd be great to be able to leverage pnpm as a default in this orb. Anywhere that I can run commands via npm or yarn, I'd like to be able to run them via pnpm.

Examples:

Ideally the pkg-manager option would allow for pnpm in addition to the existing enums.

Related: #89

[pcfreak30]

I ran into this as well and might have to fork to support it.