[FEAT] Implement design tokens

[drfarrell]

Color Variables make sure that designs comply with brand guidelines, and has been a really popular feature adopted by designers as they've become more technical.

Onlook should be able to...

• Read Variables from a rendered page or a codebase and display them for the user
• Allow users to create new Variables in a codebase
• Allow users to change existing Variables in a codebase
• Allow users to assign Variables to properties on an element

Why is this an important feature?

• It's perceived as a blocker for getting users to adopt the tool in their workflows – the current use of hex codes is insufficient for more complex codebases with variables already defined.

This understandably will require a good amount of design research to include in the current interface before it is built out.