Convert notifications policies frontend code to Typescript (#29868)

app/javascript/mastodon/reducers/notification_policy.ts

@@ -0,0 +1,18 @@
+import { createReducer, isAnyOf } from '@reduxjs/toolkit';
+
+import {
+  fetchNotificationPolicy,
+  updateNotificationsPolicy,
+} from 'mastodon/actions/notification_policies';
+import type { NotificationPolicy } from 'mastodon/models/notification_policy';
+
+export const notificationPolicyReducer =
+  createReducer<NotificationPolicy | null>(null, (builder) => {
+    builder.addMatcher(
+      isAnyOf(
+        fetchNotificationPolicy.fulfilled,
+        updateNotificationsPolicy.fulfilled,
+      ),
+      (_state, action) => action.payload,
+    );
+  });