Make some objects read-only in event-related APIs (#49045)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/49045

Changelog: [internal]

Making some objects read-only to reflect usage and allow callers to pass both read-only and writable objects.

Reviewed By: yungsters

Differential Revision: D68831136

fbshipit-source-id: e9a2d96ec0abd13f609d26d376e6da946f802011

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

@@ -19,10 +19,10 @@ import type {EventInit} from './Event';
 
 import Event from './Event';
 
-export type CustomEventInit = {
+export type CustomEventInit = $ReadOnly<{
   ...EventInit,
   detail?: mixed,
-};
+}>;
 
 export default class CustomEvent extends Event {
   _detail: mixed;

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

@@ -36,11 +36,11 @@ import {
   setStopPropagationFlag,
 } from './internals/EventInternals';
 
-export type EventInit = {
+export type EventInit = $ReadOnly<{
   bubbles?: boolean,
   cancelable?: boolean,
   composed?: boolean,
-};
+}>;
 
 export default class Event {
   static +NONE: 0;

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

@@ -39,16 +39,16 @@ export type EventHandler = interface {
 };
 export type EventListener = EventCallback | EventHandler;
 
-export type EventListenerOptions = {
+export type EventListenerOptions = $ReadOnly<{
   capture?: boolean,
-};
+}>;
 
-export type AddEventListenerOptions = {
+export type AddEventListenerOptions = $ReadOnly<{
   ...EventListenerOptions,
   passive?: boolean,
   once?: boolean,
   signal?: AbortSignal,
-};
+}>;
 
 type EventListenerRegistration = {
   +callback: EventListener,