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/DOM/Document.h

@@ -92,6 +92,8 @@ public:
     CSS::StyleSheetList& style_sheets() { return *m_style_sheets; }
     const CSS::StyleSheetList& style_sheets() const { return *m_style_sheets; }
 
+    NonnullRefPtr<CSS::StyleSheetList> style_sheets_for_bindings() { return *m_style_sheets; }
+
     virtual FlyString node_name() const override { return "#document"; }
 
     void set_hovered_node(Node*);

Userland/Libraries/LibWeb/DOM/Document.idl

@@ -29,6 +29,8 @@ interface Document : Node {
     Comment createComment(DOMString data);
     Range createRange();
 
+    [ImplementedAs=style_sheets_for_bindings] readonly attribute StyleSheetList styleSheets;
+
     readonly attribute DOMString compatMode;
     readonly attribute DocumentType? doctype;