LibWeb: Add method for listing all style sheets on a page

This will be used by the inspector, for showing style sheet contents.

Identifying a specific style sheet is a bit tricky. Depending on where
it came from, a style sheet may have a URL, it might be associated with
a DOM element, both, or neither. This varied information is wrapped in
a new StyleSheetIdentifier struct.

Userland/Libraries/LibWeb/Forward.h

@@ -200,6 +200,7 @@ class StringStyleValue;
 class StyleComputer;
 class StyleProperties;
 class StyleSheet;
+struct StyleSheetIdentifier;
 class StyleSheetList;
 class StyleValueList;
 class Supports;