LibWeb: Start exposing CSS style sheets to JavaScript :^)

This patch adds bindings for the following objects:

- StyleSheet
- StyleSheetList
- CSSStyleSheet

You can get to a document's style sheets via Document.styleSheets
and iterate through them using StyleSheetList's item() and length().

That's it in terms of functionality at this point, but still neat. :^)

Userland/Libraries/LibWeb/Forward.h

@@ -203,7 +203,7 @@ class XMLHttpRequestEventTarget;
 }
 
 namespace Web::Bindings {
-
+class CSSStyleSheetWrapper;
 class CanvasRenderingContext2DWrapper;
 class CharacterDataWrapper;
 class CommentWrapper;
@@ -301,6 +301,8 @@ class SVGGeometryElementWrapper;
 class SVGGraphicsElementWrapper;
 class SVGPathElementWrapper;
 class SVGSVGElementWrapper;
+class StyleSheetWrapper;
+class StyleSheetListWrapper;
 class TextWrapper;
 class UIEventWrapper;
 class WindowObject;