Add JSpecify annotations

[MukjepScarlet]

Hi! Currently many famous Java library including Spring, Jackson, Guava, Netty are applying JSpecify nullability controlling. As a base lib, I hope fastutil can also have an update with it! Grazie.

[vigna]

Go on with your PR :).

[MukjepScarlet]

Okay, I will try, but that requires (I guess very many) review because I'm not sure about every API's nullability

[MukjepScarlet]

Alright, current issue is I don't even know how to build... lol

[MukjepScarlet]

#375