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/HTMLLinkElement.h

@@ -53,7 +53,10 @@ private:
     virtual void resource_did_fail() override;
     virtual void resource_did_load() override;
 
-    // ^ HTMLElement
+    // ^DOM::Node
+    virtual bool is_html_link_element() const override { return true; }
+
+    // ^HTMLElement
     virtual void visit_edges(Cell::Visitor&) override;
 
     struct LinkProcessingOptions {