Please add support for UniTask

Hi,

UniTask (https://github.com/Cysharp/UniTask) is a super nice library for having async/await support for Unity, e.g. it allows to await Tasks in Coroutines or await Addressables.LoadAssetAsync etc.

You also can mark Unity's `Start` method to be a UniTask, like this:

```
private async UniTaskVoid Start() {
// ...
}
```

Rider now reports two things:

1. "Incorrect return type" which is not true (see https://github.com/Cysharp/UniTask#async-void-vs-async-unitaskvoid at the bottom of this section)
2. `Start` is never used (that's a following error from 1.)

It would be super nice, if the plugin could handle the usage of UniTask.

Thanks!