feat(data/int/basic): add lemma `int.abs_le_one_iff` (#13513)
Also renaming `int.eq_zero_iff_abs_lt_one`.
The proof is due to @Ruben-VandeVelde
Discussed on Zulip [here](https://leanprover.zulipchat.com/#narrow/stream/217875-Is-there-code-for-X.3F/topic/Integers.20of.20norm.20at.20most.20one)
Co-authored-by: Ruben Van de Velde <65514131+Ruben-VandeVelde@users.noreply.github.com>
