Refactor Immediates handling to allow for them to be unrefed, similar
to setTimeout, but without extra handles.
Document the new `immediate.ref()` and `immediate.unref()` methods.
Add SetImmediateUnref on the C++ side.
PR-URL: https://github.com/nodejs/node/pull/18139
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Franziska Hinkelmann <franziska.hinkelmann@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
