Add checks if a variant is valid for a given type

[margaretjgu]

An extension from this comment and issue

Let's add a rule that explicitly checks if a variant is valid for a given type, e.g.:

Request > none
Response > none
Interface > Container
Alias > Internal, External, Untagged
...

In this particular case the compiler should also throw an exception and refuse emitting the schema.json since this creates a situation that is not mappable to the metamodel. We do this in other situations when a tag is incorrectly used, so this is probably an oversight.