[RLC] Support namedUnion

[MaryGao]

Cadl has the named union type like below：

model SimpleModel {
  prop?: Breed;
}

union Breed {
  beagle: Beagle,
  shepherd: GermanShepherd,
  retriever: GoldenRetriever,
}

We should support this in emitter. I think we have two options here.

Option 1: Take it as general union

export interface SimpleModel {
  prop?: Beagle | GermanShepherd | GoldenRetriever;
}

Option 2: Take it as type alias (preferred)

export type Breed = Beagle | GermanShepherd | GoldenRetriever;

export interface SimpleModel {
  prop?: Breed;
}

I prefer the option 2 considering we could have a name for this union for better expression.

/cc @qiaozha @joheredi