src: 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.

Currently, `uv_default_loop()` works if the embedder is the
single-threaded default node executable, but even without
the presence of e.g. workers that might not really an API
guarantee for when Node is embedded.

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


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