LibWeb: Port custom properties to FlyString

Author: https://github.com/shannonbooth
Commit: ea2b733862
Pull-request: https://github.com/SerenityOS/serenity/pull/21828

Userland/Libraries/LibWeb/CSS/Parser/Declaration.h

@@ -19,7 +19,7 @@ public:
     Declaration(FlyString name, Vector<ComponentValue> values, Important);
     ~Declaration();
 
-    StringView name() const { return m_name; }
+    FlyString const& name() const { return m_name; }
     Vector<ComponentValue> const& values() const { return m_values; }
     Important importance() const { return m_important; }