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/Reference.h

@@ -24,7 +24,7 @@ public:
         Environment,
     };
 
-    Reference() { }
+    Reference() = default;
     Reference(BaseType type, PropertyKey name, bool strict)
         : m_base_type(type)
         , m_name(move(name))