Set up Figma sandbox for exploring dark mode styles

Purpose

We have a complete inventory of semantic design tokens via Design tokens > Token inventory (gitlab-org&13205 - closed), and guiding principles for dark mode from 🔳 Dark mode > Design exploration (&3 - closed). Now we need to verify:

• Do our principles hold up in dark mode?
• Do we need new tokens to support dark mode?

We need to evaluate these questions in a "clean" space that will not impact our published Pajamas components. With this issue, we will stand up a new Figma sandbox to continue our design vision work.

Tasks

• Create a new Figma design file for dark mode vision
• Pull design tokens into this file
• Add sample Pajamas components into this file