Persist pruned styles in LocalStorage

[sliminality]

After pruning, the resulting stylesheets should be cached somehow in LocalStorage.

PROBLEM: Coming up with a unique and workable key for each stylesheet is hard.

• StyleSheetId is not persistent
• Most CSSStyleSheetHeader info isn't uniquely identifying

   // this gives 'regular::false:0:0:undefined' for everything, basically
   `${origin}:${sourceURL}:${isInline}:${startLine}:${startColumn}:${length}` 

• Can't rely on sourceURL because of increasing popularity of inline styles

https://developer.mozilla.org/en-US/docs/Web/API/Storage/LocalStorage