Add cancellableValueTaskOption and cancellableValueTaskResult (for IcedTasks)

[cmeeren]

I find myself in need of a Option/Result CEs for the cancellable IcedTasks CEs, particularly for cancellableValueTask.

Unfortunately I can't help with this, because the builder for cancellableValueTask (which I assume must be replicated and adjusted?) is far above my level. But I thought I'd post this issue anyway.

One could of course add Option/Result CEs for all the IcedTask CEs. I'm currently only using the cancellable ones.