Autoloader plugin feature: Support callback

Information:

• Prism version: 1.16.0
• Plugins: autoloader
• Environment: All environments

Description
Right now, the autoloader plugin has no way to signal it is done working. It downloads the language as expected and highlights the piece of the code, but it doesn't have a way to support a callback function for when it is done. As you can see in the source code:
https://github.com/PrismJS/prism/blob/be909b18faa0afa899bb71ac1ab1f06c499277ce/plugins/autoloader/prism-autoloader.js#L286

Example

import { highlightElement, plugins } from 'prismjs/components/prism-core';
import 'prismjs/plugins/autoloader/prism-autoloader';
plugins.autoloader.languages_path = `https://unpkg.com/prismjs@latest/components/`;

const code = this.root.querySelector('code');
highlightElement(code, false, () => {
  alert('done highlighting');
});

In this example, the alert will show before actual highlighting is done, since the auto-loader plugin is still downloading the sources.

Start with plugins/autoloader/prism-autoloader.js around line 286 and trace how language downloads complete before highlighting. Define the callback timing so it runs only after the autoloader finishes and highlighting is complete, then verify the example no longer alerts early.