src: do not use `std::function` for `OnScopeLeave`

Using `std::function` adds an extra layer of indirection, and in
particular, heap allocations that are not necessary in our use case
here.

PR-URL: https://github.com/nodejs/node/pull/30134


Reviewed-By: Gus Caplan <me@gus.host>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: David Carlier <devnexen@gmail.com>
Reviewed-By: Franziska Hinkelmann <franziska.hinkelmann@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>