lib: add CSS global

This pull request introduces a new global variable, CSS. It aims to replicate the functionality of The CSS Web API, excluding the supports, highlight, or registerProperty properties.

By the way, I'm aware that I need to expose this global somewhere and add it to eslintrc, but I'm not sure how to do that.

(Stability: 2 - Stable | No unstable APIs are used in this class, and with it's simplicity, I believe it is stable)