Create a new #JSCValue from instance. If instance is null a new empty object is created.
When instance is provided, jsc_class must be provided too. jsc_class takes ownership of
instance that will be freed by the #GDestroyNotify passed to javascriptcore.context.Context.registerClass.
Create a new #JSCValue from instance. If instance is null a new empty object is created. When instance is provided, jsc_class must be provided too. jsc_class takes ownership of instance that will be freed by the #GDestroyNotify passed to javascriptcore.context.Context.registerClass.