Extend the assert class with two more integer checks

[Marcel Strahl]

First of all, thank you for this great package!
I've been using the package in my projects for over a year now and have had the problem of having to do negative-int and non-negative-int checks over the last few months. Psalm itself supports these "types". So I thought it would be good if there were checks analogous to integer and positive integer in this package.
Also, the two checks would complete the integer checks of the package :)
I hope you see it the same way.