Is your feature request related to a problem? Please describe.
In some cases I am trying to add files to the watcher while the watcher has already been closed. Instead of writing logic on the (OS dependent) error message I receive, I want to check whether the watcher has been closed myself before adding files.

Describe the solution you'd like
Make Watcher.isClosed public.

Describe alternatives you've considered

...
err := watcher.Add(path)
if err.Error() == "inotify instance already closed" || err.Error() == "watcher already closed" { // OS dependent
    ...
}

Additional context
Add any other context or screenshots about the feature request here.