IntoIterator for EventReader (#7720)

# Objective
Continuation of https://github.com/bevyengine/bevy/pull/5719
Now that we have a definable type for the iterator.

---

## Changelog
- Implemented IntoIterator for EventReader so you can now do `&mut reader` instead of `reader.iter()` for events.

crates/bevy_ecs/src/event.rs

@@ -241,6 +241,14 @@ impl<'w, 's, E: Event> EventReader<'w, 's, E> {
     }
 }
 
+impl<'a, 'w, 's, E: Event> IntoIterator for &'a mut EventReader<'w, 's, E> {
+    type Item = &'a E;
+    type IntoIter = ManualEventIterator<'a, E>;
+    fn into_iter(self) -> Self::IntoIter {
+        self.iter()
+    }
+}
+
 /// Sends events of type `T`.
 ///
 /// # Usage