Add compile-time validation for effectively always cancelling listeners

[PaintNinja]

The validator detects and warns for effectively never cancelling listeners, however effectively always cancelling ones have been overlooked.

@SubscribeEvent
boolean neverCancellingListener(ExampleCancellableEvent event) {
    return false; // validator suggests changing this method’s return type to void because it’s always returning false
}

@SubscribeEvent
boolean alwaysCancellingListener(ExampleCancellableEvent event) {
    return true; // it would be good if the compile-time validator detected this and suggested changing this method’s return type to void and putting @SubscribeEvent(alwaysCancelling = true)
}

[LexManos]

Leaving as a note here, as im in the weeds working on other things.

A tweaked copy of this should do it.