src: add helper for before/after scope without JS calls

Add `AsyncScope` for cases where the async_hooks `before` and
`after` callbacks should be called, to track async context,
but no actual JS is called in between and we can therefore
skip things like draining the microtask or `nextTick` queues.

PR-URL: https://github.com/nodejs/node/pull/18936


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>