[Bug]: TypeScript Emitter Framework does not handle new discriminated unions

[bterlson]

Describe the bug

Presently, it emits them just like normal unions, but it should produce proper envelopes and such depending on the options passed to @discriminated.

Reproduction

.

Checklist

• Follow our Code of Conduct
• Check that there isn't already an issue that request the same bug to avoid creating a duplicate.
• Check that this is a concrete bug. For Q&A open a GitHub Discussion.
• The provided reproduction is a minimal reproducible example of the bug.

[bterlson]

Example implementation in Zod: https://github.com/bterlson/typespec-zod/blob/main/src/chain-builders/union.tsx#L34