Add code coverage

[mhdawson]

Add code coverage for node-addon-api, ideally run for every PR.

Could re-use same approach as for Node.js core which is in the Makefile - https://github.com/nodejs/node/blob/68c4b8d56d653a77747648b933618ade63bf2007/Makefile#L227

[github-actions]

This issue is stale because it has been open many days with no activity. It will be closed soon unless the stale label is removed or a comment is made.

[github-actions]

This issue is stale because it has been open many days with no activity. It will be closed soon unless the stale label is removed or a comment is made.