n-api: refactor napi_addon_register_func

[boingoing]

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.

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• tests and/or benchmarks are included
• documentation is changed or added
• commit message follows commit guidelines

Affected core subsystem(s)

n-api