test: add .gitignore file for node-api

Refs: https://github.com/nodejs/node/pull/24557#issuecomment-444284114

PR-URL: https://github.com/nodejs/node/pull/24839


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Daniel Bevenius <daniel.bevenius@gmail.com>