n-api: add methods to open/close callback scope

Add support for the following methods;
  napi_open_callback_scope
  napi_close_callback_scope

These are needed when running asynchronous methods directly
using uv.

PR-URL: https://github.com/nodejs/node/pull/18089
Fixes: https://github.com/nodejs/node/issues/15604


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>