[React Native] Fabric get current event priority (#21553)

* Call into Fabric to get current event priority

Fix flow errors

* Prettier

* Better handle null and undefined cases

* Remove optional chaining and use ?? operator

* prettier-all

* Use conditional ternary operator

* prettier

scripts/flow/react-native-host-hooks.js

@@ -184,6 +184,9 @@ declare var nativeFabricUIManager: {
     isJsResponder: boolean,
     blockNativeResponder: boolean,
   ) => void,
+  unstable_DefaultEventPriority: number,
+  unstable_DiscreteEventPriority: number,
+  unstable_getCurrentEventPriority: () => number,
   ...
 };