LibWeb: Implement all "attributes" mutation records for MutationObserver

Author: https://github.com/Lubrsi
Commit: a718c62c01
Pull-request: https://github.com/SerenityOS/serenity/pull/14559

Userland/Libraries/LibWeb/DOM/NamedNodeMap.h

@@ -55,6 +55,8 @@ private:
     Element& associated_element() { return ref_count_target(); }
     Element const& associated_element() const { return ref_count_target(); }
 
+    void remove_attribute_at_index(size_t attribute_index);
+
     NonnullRefPtrVector<Attribute> m_attributes;
 };