Currently `LinkManagerService#currentTransitionStack()` has an implicit
return type:

```ts
export default class LinkManagerService extends Service {
  @Tracked private internalCurrentTransitionStack?: Transition[];

  get currentTransitionStack() {
    return this.internalCurrentTransitionStack;
  }
}
```

Which results in the following type declaration:

```ts
declare class LinkManagerService extends Service {
  private internalCurrentTransitionStack?;

  get currentTransitionStack(): Transition<unknown>[] | undefined;
}
```

Which causes errors when using the types from `ember-source/types`
because `Transition` is not generic.

Adding an explicit return type to `currentTransitionStack` seems to
prevent TypeScript from expanding `Transition[]` to
`Transition<unknown>[]`.

Here is the resulting type after the change:

```
declare class LinkManagerService extends Service {
  private internalCurrentTransitionStack?;

  get currentTransitionStack(): Transition[] | undefined;
}
```

I'm guessing there is a better way to deal with this but I'm not sure
what it is.