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/HTML/HTMLStyleElement.h

@@ -32,6 +32,9 @@ public:
 private:
     HTMLStyleElement(DOM::Document&, DOM::QualifiedName);
 
+    // ^DOM::Node
+    virtual bool is_html_style_element() const override { return true; }
+
     virtual void initialize(JS::Realm&) override;
     virtual void visit_edges(Cell::Visitor&) override;