[Feature Request] provide apis to tell the downstream emitters about the extensible enum info

From https://github.com/Azure/typespec-azure-pr/issues/3827 we are planning to use unions to express extensible enum, but we treat extensible enum different in the emitter compared with regular union. I wonder if compiler could provide some apis to tell the downstream emitter about if we could treat an union as extensible ? As compiler has the information about type relations https://typespec.io/docs/language-basics/type-relations#type-hierarchy

Another side question is, if we add an unrelated type into the union, which break the extensible enum rule, should we allow it ?
changing `BaseType | SubType1 | SubType2` into `BaseType | SubType1 | SubType2 | UnrelatedType`
as changing from extensible enum to union could be breaking for emitters. Also, should we add linter rule for that ?

Another reason, I think compiler should provide the detail is that I think the above discussion is for both branding and non-branding scope ? If yes, it would probably not be appropriate for tcgc to provide the apis.