[compiler] Allow `@discriminated` union variants to be unions when `envelope` is "none"

### Clear and concise description of the problem

Currently, in `@discriminated(#{ envelope: "none"})` unions, we require variants to be _models_ that have the discriminator property on them set to the correct value (`Error: invalid-discriminated-union-variant` if not). I propose that we allow a discriminated union variant to also be a union if each variant of the sub-union conforms to the discriminator property rule. This would allow constructs like the following:

```tsp
@discriminated(#{ envelope: "none", discriminatorPropertyName: "kind" })
union X {
a: A;
b: B;
}

model A {
kind: "a";
//...
}

@discriminated(#{ envelope: "none", discriminatorPropertyName: "bKind" })
union B {
c: C;
d: D;
}

model C {
kind: "b";
bKind: "c";
// ...
}

model D {
kind: "b";
bKind: "d";
// ...
}
```

I ran into this trying to describe a schema with this sort of structure, where one of the variants of a top level union discriminated by "kind" itself has several subvariants discriminated by its own separate property.

### Checklist

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- [x] Read the [docs](https://typespec.io/docs/).
- [x] Check that there isn't already an issue that request the same feature to avoid creating a duplicate.