LibCore: Add a way to check if a property is readonly

Author: https://github.com/alimpfard
Commit: 2b38b2f83b
Pull-request: https://github.com/SerenityOS/serenity/pull/4759
Reviewed-by: https://github.com/ADKaster

Libraries/LibCore/Property.h

@@ -48,6 +48,7 @@ public:
     JsonValue get() const { return m_getter(); }
 
     const String& name() const { return m_name; }
+    bool is_readonly() const { return !m_setter; }
 
 private:
     String m_name;