fix: restore instance-level Event phase declarations without readonly

Flow needs the instance-level field declarations (NONE, CAPTURING_PHASE,
AT_TARGET, BUBBLING_PHASE) to know these properties exist on Event
instances. The previous commit removed them entirely, causing 12 Flow
prop-missing errors. This restores them without the `readonly` modifier
so Hermes won't create non-writable properties that conflict with the
Object.defineProperty calls.

Co-authored-by: Cursor <cursoragent@cursor.com>

Author: IsaacIsrael

packages/react-native/src/private/webapis/dom/events/Event.js

@@ -50,6 +50,11 @@ export default class Event {
   static readonly AT_TARGET: 2;
   static readonly BUBBLING_PHASE: 3;
 
+  NONE: 0;
+  CAPTURING_PHASE: 1;
+  AT_TARGET: 2;
+  BUBBLING_PHASE: 3;
+
   _bubbles: boolean;
   _cancelable: boolean;
   _composed: boolean;