[Feature] Support double nullable types

[mohammadfawaz]

Allow types to be double nullable as in

let x: u8?? = 1;

This is rarely useful but it can sometimes be.

[vicsn]

Not a fan (because I havent used it in the wild before)

[mohammadfawaz]

Yeah I agree. I will keep this open for completeness but I doubt we'll get it any time soon.