Libraries: Use default constructors/destructors in LibJS

https://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#cother-other-default-operation-rules

"The compiler is more likely to get the default semantics right and
you cannot implement these functions better than the compiler."

Userland/Libraries/LibJS/Runtime/PropertyKey.h

@@ -37,7 +37,7 @@ public:
         return TRY(value.to_string(global_object));
     }
 
-    PropertyKey() { }
+    PropertyKey() = default;
 
     template<Integral T>
     PropertyKey(T index)