Add Firefox and WebKit to design systems

[lukewarlow]

We currently have "web" and "Chromium" (with only 1 item). This issue is to track adding Firefox and WebKit as design systems and documenting the various elements including their pseudo elements. Along with doing the same to the Chromium entry.

https://gist.github.com/lukewarlow/2eeb75ba6d3039cd10d850c19162976b is a start on this research and I now intend to transfer it across to the Open UI site in a way that can be fed into research pages.