feat: enable registration of PropertyTarget DesignToken roots

[nicholasrice]

Pull Request

📖 Description

This PR expands the structures assignable as DesignToken roots to include the 'PropertyTarget' types. This will facilitate SSR scenarios by allowing authors to register a PropertyTarget structure that can easily stringify the DesignToken CSS custom property values for the compilation context, injecting them into the SSR result wherever is appropriate.

🎫 Issues

closes #6271

👩‍💻 Reviewer Notes

📑 Test Plan

✅ Checklist

General

• [ ] I have included a change request file using $ yarn change
• [ ] I have added tests for my changes.
• [ ] I have tested my changes.
• [ ] I have updated the project documentation to reflect my changes.
• [ ] I have read the CONTRIBUTING documentation and followed the standards for this project.

Component-specific

• [ ] I have added a new component
• [ ] I have modified an existing component
• [ ] I have updated the definition file
• [ ] I have updated the configuration file

⏭ Next Steps