fix(types): better ember-concurrency@v1 support in typescript

Issue: #297

Resolves this issue:
```
Argument of type 'TaskForAsyncTaskFunction<() => Promise<Food>>'
  is not assignable to parameter of type 'TaskIsh<Food, Data[]>'.

  The types returned by 'perform(...).cancel(...)' are incompatible between these types.
    Type 'void' is not assignable to type 'Promise<void>'.
```

ember-resources/src/-private/resources/ember-concurrency-task.ts

@@ -34,7 +34,7 @@ export interface TaskInstance<Return = unknown> extends Promise<Return> {
   readonly isFinished: boolean;
   readonly isRunning: boolean;
   readonly isDropped: boolean;
-  cancel(reason?: string): Promise<void>;
+  cancel(reason?: string): void | Promise<void>;
 }
 
 // @private