fs: add noop stub for FSWatcher.prototype.start

* Motivation: In a previous PR, #29905, I made this method a private
method since it had no value to the user.

There was discussion that maybe it should have been a runtime
deprecation first, but was ultimatley decided that for this
type of method, a noop stub was a better option.

This Adds back in the method, but as a noop stub, while also keeping
the real implementation private

PR-URL: https://github.com/nodejs/node/pull/30160


Reviewed-By: David Carlier <devnexen@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>