Add pnpm store caching

[jrmajor]

pnpm can install Node runtime as a dependency, but actions/setup-node is still needed to cache the pnpm store (as currently suggested in the readme). Alternative is to add something like this to your workflow:

- name: Get pnpm store directory path
  id: pnpm-cache-dir
  run: echo "dir=$(pnpm store path --silent)" >> $GITHUB_OUTPUT

- name: Cache pnpm dependencies
  uses: actions/cache@v4
  with:
    path: ${{ steps.pnpm-cache-dir.outputs.dir }}
    key: pnpm-${{ hashFiles('pnpm-lock.yaml') }}
    restore-keys: pnpm-

With this PR, the same can be accomplished simply by setting cache: true in setup pnpm action:

- name: Setup pnpm
  uses: pnpm/action-setup@v4
  with:
    cache: true

The implementation is based on actions/setup-node.

Closes #70, closes #80, closes #82, closes #152, closes #173, closes #185.

Once stable, this could be enabled by default, although I think it would require a major version bump.

[oBusk]

Would also close #185

[jrmajor]

Thanks @oBusk, linked that issue too.

[jrmajor]

If you would like to test this in your workflows, you can use the hash of the latest commit from this branch as a version:

- name: Setup pnpm
  # https://github.com/pnpm/action-setup/pull/188
  uses: pnpm/action-setup@0bc9da32cd356746083d9e2b8848bad8b974c72c
  with:
    cache: true