src: make `AsyncResource` destructor virtual

`AsyncResource` is intended to be a base class, and since we don’t
know what API consumers will do with it in their own code,
it’s good practice to make its destructor virtual.

This should not be ABI-breaking since all class methods are inline.

PR-URL: https://github.com/nodejs/node/pull/20633


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Daniel Bevenius <daniel.bevenius@gmail.com>
Reviewed-By: Tiancheng "Timothy" Gu <timothygu99@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>