n-api: Reference and external tests

 - Add a test project to addons-napi that covers the
   N-API reference and external APIs
 - Fix a bug in napi_typeof that was found by the new tests

PR-URL: https://github.com/nodejs/node/pull/12551


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>