Add pnpm package manager support

[ivanovac]

Summary

Adds pnpm support to the Node.js buildpack for applications containing pnpm-lock.yaml

Changes

• Installs the manifest-provided pnpm dependency during staging.
• Adds pnpm to the buildpack dependency PATH.
• Runs pnpm install during dependency resolution.
• Supports online and cached/offline pnpm installs.
• Adds pnpm cache handling through .pnpm-store.
• Reports pnpm-specific unmet dependency warnings.
• Updates pnpm fixtures to lockfile format 9.
• Adds integration coverage for:
  • Standard pnpm applications
  • Out-of-date lockfiles
  • Unmet dependencies
  • pnpm workspaces
  • Development dependencies
• Adds the pnpm dependency to manifest.yml

The dependency pipeline and binary-builder changes select and package the correct pnpm-linux-x64.tar.gz release asset as an any-stack dependency.

Testing

Docker integration coverage passed for all four pnpm scenarios on cflinuxfs4

[tnikolova82]

lgtm