n-api: add napi_get_version

Add napi_get_version function so that addons can
query the level of N-API supported.

PR-URL: https://github.com/nodejs/node/pull/13207
Fixes: https://github.com/nodejs/abi-stable-node/issues/231


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Jason Ginchereau <jasongin@microsoft.com>