Convert Space and Depth to Rem

[myvuuu]

🚀 Feature Proposal

• [ ] Convert depth and space tokens to rem
• [ ] Add documentation on why we're making the change, how it'll impact users, and what they need to do in the migration docs

Open Q's:

• How many teams are using their own rems vs. pixels? Only one team using their own rem for a backend application
• For each usage scenario, how will introducing rem based depth and space tokens impact product teams? N/A

[myvuuu]

Still need to determine how changing to rem will impact teams (might need a discovery ticket)