n-api: add ability to remove a wrapping

Calling napi_wrap() twice on the same object has the result of returning
napi_invalid_arg on the second call. However, sometimes it is necessary
to replace the native pointer associated with a given object. This new
API allows one to remove an existing pointer, returning the object to
its pristine, non-wrapped state.

PR-URL: https://github.com/nodejs/node/pull/14658


Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>
Fixes: https://github.com/nodejs/abi-stable-node/issues/266