Automatically detects all S3 classes that have a specialized notify() method
(like notify.MyNotifier) and prints them as a character vector of class names
(e.g., "PushbulletNotifier", "TelegramNotifier").
Note that only classes defined and loaded at the time you call this function will
appear. If you just created a new notifier in another file or package, ensure it's
sourced/loaded first.