feat: support nested subpaths

[pi0]

🔗 Linked issue

Context: #251

❓ Type of change

• 📖 Documentation (updates to the documentation, readme, or JSdoc annotations)
• 🐞 Bug fix (a non-breaking change that fixes an issue)
• 👌 Enhancement (improving an existing functionality like performance)
• ✨ New feature (a non-breaking change that adds functionality)
• 🧹 Chore (updates to the build process or auxiliary tools and libraries)
• ⚠️ Breaking change (fix or feature that would cause existing functionality to change)

📚 Description

This PR adds support for nested subpaths (in a directory) for rollup builds (both automated and custom supported)

See implementation notes in #251 (comment)

📝 Checklist

• I have linked an issue or discussion.
• I have updated the documentation accordingly.