Bug #2845 typescript angular inheritance (#3812)

* issue #2845: enable 'supportsMultipleInheritance' on typescript angular client codegen

- note I reran ./bin/openapi3/typescript-angular-petstore-all.sh and no changes occurred.
  this suggests to me that the petstore.yaml sample should be improved to make use of the
  anyOf / allOf / oneOf keywords, in order to better show the effects of changes on generated code.

* issue #2845: run ./bin/openapi3/typescript-angular-petstore-all.sh

* run `mvn clean package && ./bin/typescript-angular-petstore-all.sh`

* revert extranous files

modules/openapi-generator/src/main/java/org/openapitools/codegen/languages/TypeScriptAngularClientCodegen.java

@@ -67,6 +67,8 @@ public TypeScriptAngularClientCodegen() {
         super();
         this.outputFolder = "generated-code/typescript-angular";
 
+        supportsMultipleInheritance = true;
+
         embeddedTemplateDir = templateDir = "typescript-angular";
         modelTemplateFiles.put("model.mustache", ".ts");
         apiTemplateFiles.put("api.service.mustache", ".ts");