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: #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>

lib/internal/fs/watchers.js

@@ -174,6 +174,12 @@ FSWatcher.prototype[kFSWatchStart] = function(filename,
   }
 };
 
+// To maximize backward-compatiblity for the end user,
+// a no-op stub method has been added instead of
+// totally removing FSWatcher.prototpye.start.
+// This should not be documented.
+FSWatcher.prototype.start = () => {};
+
 // This method is a noop if the watcher has not been started or
 // has already been closed.
 FSWatcher.prototype.close = function() {