fs: Add noop stub for FSWatcher.prototype.start

* Motivation: In a previous PR, nodejs#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

lib/internal/fs/watchers.js

@@ -174,6 +174,8 @@ FSWatcher.prototype[kFSWatchStart] = function(filename,
   }
 };
 
+FSWatcher.prototype.start = () => {};
+
 // This method is a noop if the watcher has not been started or
 // has already been closed.
 FSWatcher.prototype.close = function() {