Refactor teal.transform (part 3)

[kumamiao]

A continuation of #107

Tasks

• Finalize PoC feature branch (need to list what else needs to do). ✅
• Demo to teal champions, Daniel, Joe, Pawel, Mariia
• Gather feedback or implement feedback
• Finalize and write unit tests
• Write vignette
• Start Pre-release candidate
• Release
• [Feature Request]: variables with interaction variables teal.transform#279

[donyunardi]

Due to other priorities, we didn't get to do this in 2025 PI4. We will setup demo on 2026 PI 1.

From what I understand, the PoC is ready to be shown.