Add support for volta.pnpm field

[simonbuchan]

actions/setup-node@v4 now supports reading volta.node / volta.extends from node-version-file: package.json, it would be nice for pnpm/action-setup to support volta.pnpm too.

Volta currently only has experimental support for pnpm, but from extended use the only reason for this seems to be that they don't support global installs (which is hopefully not a big use case)

Presumably for compatibility packageManager should still be checked first, and I'm not particularly concerned about support for volta.extends.

[NullVoxPopuli]

👋 I added this here: https://github.com/wyvox/action-setup-pnpm