node-api: allow retrieval of add-on file name

Unlike JS-only modules, native add-ons are always associated with a
dynamic shared object from which they are loaded. Being able to
retrieve its absolute path is important to native-only add-ons, i.e.
add-ons that are not themselves being loaded from a JS-only module
located in the same package as the native add-on itself.

Currently, the file name is obtained at environment construction time
from the JS `module.filename`. Nevertheless, the presence of `module`
is not required, because the file name could also be passed in via a
private property added onto `exports` from the `process.dlopen`
binding.

As an attempt at future-proofing, the file name is provided as a URL,
i.e. prefixed with the `file://` protocol.

Fixes: https://github.com/nodejs/node-addon-api/issues/449
PR-URL: https://github.com/nodejs/node/pull/37195


Co-authored-by: Michael Dawson <mdawson@devrus.com>
Reviewed-By: Michael Dawson <midawson@redhat.com>