Better type for ValueSet._system

Add link to zulip thread with information about the contained code
system extension.

src/export/ValueSetExporter.ts

@@ -76,8 +76,10 @@ export class ValueSetExporter {
             .replace(/^([^|]+)/, csMetadata?.url ?? '$1')
             .split('|');
           composeElement.system = foundSystem[0];
-          // if the code system is also a contained resource, add the special extension
-          // if it's not a contained resource, and the system we found is an inline instance, that's a problem
+          // if the code system is also a contained resource, add the valueset-system extension
+          // this zulip thread contains a discussion of the issue and an example using this extension:
+          // https://chat.fhir.org/#narrow/stream/215610-shorthand/topic/Contained.20code.20system.20in.20the.20value.20set/near/424938537
+          // additionally, if it's not a contained resource, and the system we found is an inline instance, that's a problem
           const containedSystem = valueSet.contained?.find((resource: any) => {
             return resource?.id === csMetadata.id && resource.resourceType === 'CodeSystem';
           });

src/fhirtypes/ValueSet.ts

@@ -1,6 +1,14 @@
 import sanitize from 'sanitize-filename';
 import { Meta } from './specialTypes';
-import { Narrative, Resource, Identifier, CodeableConcept, Coding, Extension } from './dataTypes';
+import {
+  Narrative,
+  Resource,
+  Identifier,
+  CodeableConcept,
+  Coding,
+  Extension,
+  Element
+} from './dataTypes';
 import { ContactDetail, UsageContext } from './metaDataTypes';
 import { HasName, HasId } from './mixins';
 import { applyMixins } from '../utils/Mixin';
@@ -82,9 +90,7 @@ export type ValueSetCompose = {
 
 export type ValueSetComposeIncludeOrExclude = {
   system?: string;
-  _system?: {
-    extension?: Extension[];
-  };
+  _system?: Element;
   version?: string;
   valueSet?: string[];
   concept?: ValueSetComposeConcept[];