lib,src: make `StatWatcher` a `HandleWrap`

Wrapping libuv handles is what `HandleWrap` is there for.
This allows a decent reduction of state tracking machinery
by moving active-ness tracking to JS, and removing all
interaction with garbage collection.

Refs: https://github.com/nodejs/node/pull/21093

PR-URL: https://github.com/nodejs/node/pull/21244


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Anatoli Papirovski <apapirovski@mac.com>
Reviewed-By: Tiancheng "Timothy" Gu <timothygu99@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>