Support reading `.tool-versions` for pnpm version

[evandam]

Hi folks 👋

It would be great if this action could support asdf's (and Mise's) .tool-versions file to get the version of pnpm to install, similar to actions/setup-node's node-version-file: .tool-versions rather than having to hardcode it in the action config or in cases where packageManager is not set in package.json.

Hopefully a small change but a nice quality-of-life improvement for at least a few users!

[zkochan]

IMO we should only actually check the "devEngines.packageManager" field in "package.json", which is now the standard way of declaring package managers and is also recognized by npm CLI.