Add string-union FhirResourceType

[GinoCanessa]

Added a string-union of known resources named FhirResourceType. Per discussion on Zulip.

[jdjkelly]

[ir4y]

@GinoCanessa
I think there is a simpler way to define FhirResourceType rather than codegen.

interface Patient {
  readonly resourceType: 'Patient'
}

interface Practitioner {
  readonly resourceType: 'Practitioner'
} 

type FhirResource = Patient | Practitioner

type FhirResourceType = FhirResource['resourceType']

https://www.typescriptlang.org/play?ssl=12&ssc=1&pln=1&pc=1#code/JYOwLgpgTgZghgYwgAgApzMC5kG8CwAUMslBHACYD2IANgJ6kQDOVArlEgCr0AOEALmQBydJmxhhRAL5EioSLEQpUURJkw1oeIiTKUaDJqw7c+gkavXBNIaFMLTkcwmHPIAYgAtgUAEos7JwoALxoGFg4AD5oaggawFpQLm78nj7+gaYQPGlh3r4BJsEA2sJkxWb8wgC6LkRAA