fs: make FSWatcher.start private

* This is a semver-major change to rename the FSWatcher.start
function to FSWatcher._start to make it private

The motivation here is that it serves no purpose to the end user.
An instance of FSWatcher is returned when a user calls fs.watch,
which will call the start method.  A user can't create an instance
of a FSWatcher directly.  If the start method is called by a user
it is a noop since the watcher has already started.  Calling start
after a watcher has closed is also a noop

PR-URL: https://github.com/nodejs/node/pull/29905


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>