core: hle: kernel: KThread: Integrate with KWorkerTask and implement DoWorkerTaskImpl.

- This is used to terminate a thread asynchronously after it has been exited.
- This fixes a crash that can occur in Pokemon Sword/Shield because a thread is incorrectly closed on svcExitThread, then, the thread is destroyed on svcCloseHandle while it is still scheduled.
- Instead, we now wait for the thread to no longer be scheduled on all cores before destroying it from KWorkerTaskManager, which is accurate to HOS behavior.