Add Web Preferences API …maybe eventually

[Schweinepriester]

• via https://www.bram.us/2024/04/13/what-if-you-had-real-control-over-light-mode-dark-mode-on-a-per-site-basis/
• https://wicg.github.io/web-preferences-api/
• https://github.com/WICG/web-preferences-api
• https://chromestatus.com/feature/4631882616012800
• Web Preferences API mozilla/standards-positions#879
• Web Preferences API WebKit/standards-positions#252

This spec requires large amounts of work to section it and word it in ways that are normative. It is currently a very rough draft.

Hence "…maybe eventually"