Reorganize the Codex design tokens into 3 tiers per ADR 8.
- Option tokens for a given theme (right now we only support one theme, wikimedia-ui)
- Decision tokens for a given mode (right now we only have a default light mode, but soon we'll have a dark mode as well; we may also define other modes or variants here as needed)
- Component-specific tokens, which should not need to change on a per-theme or per-mode basis