fix: reset listenerCount when invalidating RCTEventEmitter (#54809)

Summary:
If a subclass of RCTEventEmitter outlives the bridge and the bridge is invalidated, the invalidate method is called on RCTEventEmitter which calls stopObserving but does not change the listenerCount. Once the bridge is recreated and a listener added to the RCTEventEmitter if the listenerCount is > 1 startListening is never called: https://github.com/facebook/react-native/blob/a9a02689533f9cf30fc20cfc7be1ade4f4a624da/packages/react-native/React/Modules/RCTEventEmitter.m#L109

This just resets the listenerCount to prevent this.

Probably not a good idea to have modules that outlive bridge in first place but this seems like unwanted behavior in any case :).

## Changelog:

[iOS] [Fixed] - fix listenerCount not resetting for RCTEventEmitter

Pull Request resolved: #54809

Test Plan:
I tested this code via a patch in the artsy/eigen app repo: artsy/eigen#13068
The app was using a native module that posts events over the bridge to handle push notifications.

Behavior before:
1. Start app listeners added to ARNotificationManager module, startListener called listenerCount = 2
2. Force a dev reload invalidating the bridge
3. Invalidate called on bridge and the native module causing stopObserving to be called but listenerCount remains 2
4. Bridge refreshes listeners added again but startObserving never called

Behavior after:
1. Start app listeners added to ARNotificationManager module, startListener called listenerCount = 2
2. Force a dev reload invalidating the bridge
3. Invalidate called on bridge and the native module causing stopObserving to be called but listenerCount reset to 0
4. Bridge refreshes listeners added again listeners added again, startObserving called after first

Reviewed By: cipolleschi

Differential Revision: D88647586

Pulled By: javache

fbshipit-source-id: 8a8b8b8cf4c7e683f02a08e40fc9d95c8ba4e4ac

Author: brainbicycle

packages/react-native/React/Modules/RCTEventEmitter.m

@@ -89,6 +89,7 @@ - (void)invalidate
 
   if (_listenerCount > 0) {
     [self stopObserving];
+    _listenerCount = 0;
   }
 }