Maybe we don't need documents after all

[domenic]

Related to #57.

At TPAC we discussed what exactly ties stylesheets to documents, if anything. Our conclusion was that it's only the base URL, which is set at construction time. So in this sense document.createCSSStyleSheetSync() is equivalent to a hypothetical window.createCSSStyleSheetSync({ baseURL: document.baseURI }) or new CSSStyleSheet({ baseURL: document.baseURI }).

Given this, I suggest we just infer the base URL at construction time, in the same way many other web platform APIs do. Namely we can use the current settings object's API base URL.