Iterating over internal::registry_context elements

For a long time, I have been using `entt::registry`'s `internal::registry_context` to manage singleton components in my engine. For the serialization part, I need to iterate over all elements in that context, then run my serialization code. This is not a major issue, as I can workaround this by storing their id in a vector, then fetching each singleton component while serializing the scene, but I believe being able to iterate would still be beneficial