n-api: add napi_get_node_version

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

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• tests and/or benchmarks are included
• documentation is changed or added
• commit message follows commit guidelines

Affected core subsystem(s)

N-API

/cc @nodejs/n-api