fs: do not crash when using a closed fs event watcher

Before this commit, when the user calls methods on a closed or
errored fs event watcher, they could hit a crash since the
FSEventWrap in C++ land may have already been destroyed with
the internal pointer set to nullptr. This commit makes sure
that the user cannot hit crashes like that, instead the
methods calling on a closed watcher will be noops.

Also explicitly documents that the watchers should not be used
in `close` and `error` event handlers.

PR-URL: https://github.com/nodejs/node/pull/20985
Fixes: https://github.com/nodejs/node/issues/20738
Fixes: https://github.com/nodejs/node/issues/20297


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Ron Korving <ron@ronkorving.nl>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Tiancheng "Timothy" Gu <timothygu99@gmail.com>