Add support for NIF version 2.18 (OTP 29)

[Hermanverschooten]

Summary

Adds support for NIF version 2.18, which ships with Erlang/OTP 29.

OTP 29's erl_nif.h bumped the NIF interface to 2.18 (ERL_NIF_MAJOR_VERSION 2 / ERL_NIF_MINOR_VERSION 18); OTP 26–28 remained on 2.17. Without 2.18 in the available list, projects upgrading to a rustler that exposes the nif_version_2_18 feature can't publish or select 2.18 precompiled artifacts.

The forward-compatibility resolver (find_compatible_nif_version/2) already handles running on a newer NIF than what's published, so no logic change was needed — only adding 2.18 to the allow-list.

The default NIF version is unchanged (2.15), so existing consumers are unaffected — 2.18 is opt-in via nif_versions:.

[philss]

Awesome! Thank you, @Hermanverschooten! Later today I'm going to release a new version.