fs: make FSStatWatcher.start private

An instance of FSStatWatcher is returned when a user calls fs.watchFile,
which will call the start method. A user can't create an instance
of a FSStatWatcher directly. If the start method is called by a user
it is a noop since the watcher has already started.

This "Class" is currently undocumented.

PR-URL: https://github.com/nodejs/node/pull/29971


Reviewed-By: David Carlier <devnexen@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Michaël Zasso <targos@protonmail.com>