n-api: refactor napi_addon_register_func

As per discussion in abi-stable-node:
nodejs/abi-stable-node#256,
take a refactor to napi_addon_register_func such that
the result from the register function is assigned to
the module exports property. By making this change,
native module can be agnostic about which type of
module the environment supports.

PR-URL: nodejs/node#15088
Reviewed-By: Gabriel Schulhof <gabriel.schulhof@intel.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>
Reviewed-By: Hitesh Kanwathirtha <digitalinfinity@gmail.com>

Author: boingoing