LibWeb: Implement the cloning steps for <template> elements

Author: https://github.com/Lubrsi
Commit: a7fa757dd1
Pull-request: https://github.com/SerenityOS/serenity/pull/8449

Userland/Libraries/LibWeb/HTML/HTMLTemplateElement.h

@@ -21,6 +21,8 @@ public:
     NonnullRefPtr<DOM::DocumentFragment> content() { return *m_content; }
     const NonnullRefPtr<DOM::DocumentFragment> content() const { return *m_content; }
 
+    virtual void cloned(Node& copy, bool clone_children) override;
+
 private:
     DOM::Document& appropriate_template_contents_owner_document(DOM::Document&);