src: refactor async callback handling

- Merge the two almost-but-not-quite identical `MakeCallback()`
  implementations
- Provide a public `CallbackScope` class for embedders in order
  to enable `MakeCallback()`-like behaviour without tying that
  to calling a JS function

PR-URL: https://github.com/nodejs/node/pull/14697


Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>