Mark the custom token entity type as internal

We should not expect the Custom Token content entity type to show up in things like lists of available entity types in a reference field, or supported by XML sitemap module as a linkable item (since they are not public). I would propose adding internal = true, to the annotation of the entity type so that it will be hidden in situations like this. See #2936714: Entity type definitions cannot be set as 'internal' → for more information.