fix: add helper method and parameter to user store service

Author: efirst91

apps/angular/6-structural-directive/src/app/user.store.ts

@@ -6,10 +6,14 @@ import { User } from './user.model';
   providedIn: 'root',
 })
 export class UserStore {
-  private user = new BehaviorSubject<User | undefined>(undefined);
+  private readonly user = new BehaviorSubject<User | undefined>(undefined);
   user$ = this.user.asObservable();
 
-  add(user: User) {
+  add(user: User | undefined) {
     this.user.next(user);
   }
+
+  get currentUser(): User | undefined {
+    return this.user.getValue();
+  }
 }