Add support for CSS CustomState

[p-ob]

Spec: https://wicg.github.io/custom-state-pseudo-class/#customstateset

My team exposes custom CSS state pseudoclasses for applications to consume, and being able to document these in a standardized way would be awesome. I assume its schema would be quite similar to CSS Parts:

• name
• summary
• description
• deprecated

While currently only supported in Chromium-based browsers, hopefully something that gains traction in 2023 with further interop efforts!