n-api: make napi_get_property_names return strings

The documentation says that this method returns an array of strings.
Currently, it does not do so for indices. Resolve that by telling
V8 explicitly to convert to string.

PR-URL: https://github.com/nodejs/node/pull/27524
Fixes: https://github.com/nodejs/node/issues/27496


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Minwoo Jung <minwoo@nodesource.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>