UnsafeRegister polyfill

[Akeit0]

Why not this implementation is used? Is there any problem?
https://github.com/Cysharp/UniTask/blob/master/src/UniTask/Assets/Plugins/UniTask/Runtime/CancellationTokenExtensions.cs#L107-L127

https://github.com/Cysharp/R3/blob/main/src/R3/Internal/Shims/CancellationTokenExtensions.cs

[neuecc]

Since R3 don't heavily use async/await, I thought it might not be that important to be conscious of it, so I may have omitted it.
Indeed, maybe we should add SuppressFlow as well.