Additional properties whose structure depends entirely on the host application, with the caveat
that the Viv runtime expects a plain object all the way down. As such, the values embedded here
should not include types like functions or members of custom classes, but rather exclusively the
types defined in the ExpressionValue union.
A read-only entity view for a location in a simulated storyworld.
For details on semantics and constraints, see EntityView, which this interface extends.