Replace includeSubtypes with @Polymorphic

The new @Polymorphic annotation serves the exact same purpose as
@Event(includeSubtypes = true), but should be easier to read in complex
handler declarations. It has to be used in conjunction with the @Event
annotation, not instead of it.

event-bus-core/src/main/java/dev/kske/eventbus/core/EventBus.java

@@ -104,7 +104,7 @@ public final class EventBus {
 		for (var binding : bindings.entrySet())
 			if (binding.getKey().isAssignableFrom(eventClass))
 				for (var handler : binding.getValue())
-					if (handler.includeSubtypes())
+					if (handler.isPolymorphic())
 						handlers.add(handler);
 
 		return new ArrayList<>(handlers);