Make FHIRSchema types more strict

[ryukzak]

Problem: currently FHIRSchema type is too careless. E.g.: package_meta is optional, url and name are represented by string.

In codegen we fix it by intermediate type:

export type Name = string & { readonly __brand: unique symbol };
export type CanonicalUrl = string & { readonly __brand: unique symbol };

export interface PackageMeta {
    name: string;
    version: string;
}

export type RichFHIRSchema = Omit<FS.FHIRSchema, "package_meta" | "base" | "name" | "url"> & {
    package_meta: PackageMeta;
    name: Name;
    url: CanonicalUrl;
    base: CanonicalUrl;
};

• Do we need to add package_meta to FHIRSchema as a part of standard?
• Do we to make more specific types for FHIRSchema fields in TS?

[octoshikari]

For brand types let add globally available type helper

type Brand<K, T> = K & { __brand: T }