n-api: expose n-api version in process.versions

Expose n-api version in process.versions so that it is
available for use in javascript by external modules
like node-pre-gyp. It was previously accessible through
a functon available in the N-API.

PR-URL: https://github.com/nodejs/node/pull/18067


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Tiancheng "Timothy" Gu <timothygu99@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Minwoo Jung <minwoo@nodesource.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: Tobias Nießn <tniessen@tnie.de>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>