mismatch between docs and type returned for timer tasks

🐛 **Describe the bug**
per #322, the type hinting was not updated to match a type that has the `is-cancelled` property and `cancel()` method. The patch otherwise works beautifully.

🤔 **Expected behavior**
Just some intellisense, no functional issues

Thanks for the work on this repo!

该 issue 未列出任何文件或测试。先查看 #322 中的 type-hinting 更改，并定位 timer-task 的返回类型；确认它为 IntelliSense 暴露 is-cancelled 和 cancel()，然后在确定了相关现有测试的情况下运行这些测试。

由索引模型根据 Issue 内容生成。