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.

[mAAdhaTTah]

A hook for that seems like a viable path forward. Will take a look at that.

[RunDevelopment]

Autoloader loads language definitions and then highlights the element again, so I think that the current behavior of Prism, while not ideal, is correct.

It wouldn’t be too hard to implement as Autoloader-specific solution but we have other plugins that are also implemented by re-highlighting elements, so I’d personally prefer a unified solution. Thoughts? @mAAdhaTTah @Golmote

@nicojs Also, fetching some languages might fail. How should Prism behave in that case?