n-api: add napi_get_node_version

Add `napi_get_node_version`, to help with feature-detecting
Node.js as an environment.

Backport-PR-URL: https://github.com/nodejs/node/pull/19447
PR-URL: https://github.com/nodejs/node/pull/14696


Reviewed-By: Kyle Farnung <kfarnung@microsoft.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>