Rewrite markers reducer in Typescript (#27644)

app/javascript/mastodon/reducers/markers.ts

@@ -0,0 +1,18 @@
+import { createReducer } from '@reduxjs/toolkit';
+
+import { submitMarkersAction } from 'mastodon/actions/markers';
+
+const initialState = {
+  home: '0',
+  notifications: '0',
+};
+
+export const markersReducer = createReducer(initialState, (builder) => {
+  builder.addCase(
+    submitMarkersAction.fulfilled,
+    (state, { payload: { home, notifications } }) => {
+      if (home) state.home = home;
+      if (notifications) state.notifications = notifications;
+    },
+  );
+});