Collaborate with DefinitelyTyped fhir version folks

[jdjkelly]

• There is an existing @types/fhir project
• Can we support each other?
• Evidence that they were trying to support multiple versions
• There interfaces seem better developed, but there isn't source code to see how it was generated, or to create new versions (were they done by hand?) - not much evidence for how they are built

See these extremely relevant issues:

DefinitelyTyped/DefinitelyTyped#49719
DefinitelyTyped/DefinitelyTyped#49574

@bkaney 👋 I see you might be the most recent person to have thought about TypeScript definitions for FHIR. I think we might be able to help each other (see this repo for context).