Add Support for Inertia Router to Automatically Announce Page Titles on Navigation

[linyuqing222]

This pull request introduces a new feature that enables the Inertia.js router to automatically announce page titles during navigation, similar to how vue-router operates with vue-announcer.

The changes primarily involve modifications to Announcer.vue, where we now check if the router object has an 'afterEach' method (indicative of vue-router) or an 'on' method (indicative of Inertia router). Depending on the method available, the router will announce the page title on each navigation event.

Here's a usage example with Inertia:

import { createApp } from 'vue'
import App from './App.vue'
import { router } from '@inertiajs/vue3'
import VueAnnouncer from '@vue-a11y/announcer'
import '@vue-a11y/announcer/dist/index.css'

createApp(App)
  .use(VueAnnouncer, { router })
  .mount('#app')
// Page title is announced automatically

I'm not sure how to better implement this feature with Inertia...
Looking forward to your feedback.