Consider allowing Alias to hold code values (or consider other approaches to friendlier code handling)

It's not uncommon to need to reference the same code in multiple places throughout an IG. Currently, SUSHI requires you to repeat the formal coded value everywhere you need it (e.g., `#12345`). This can lead to mistakes if someone incorrectly types the code or if the code is changed in one place but not in others (i.e., unintentional misalignment).

We should consider allowing `Alias:` to also substitute code types.

For the full discussion, see: https://chat.fhir.org/#narrow/stream/215610-shorthand/topic/Defining.20aliases.20for.20codes.3F