n-api: add helper for addons to get the event loop

Add a utility functions for addons to use when they need
a reference to the current event loop.

While the libuv API is not directly part of N-API, it
provides a quite stable C API as well, and is tightly
integrated with Node itself.

As a particular use case, without access to the event loop
it is hard to do something interesting from inside a N-API
finalizer function, since calls into JS and therefore virtually
all other N-API functions are not allowed.

PR-URL: https://github.com/nodejs/node/pull/17109


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>