[drupalImage] Announce image upload for screenreader users

Problem/Motivation

#3222757: [drupalImage] Make image alt text required or strongly encouraged → follow-up to which displays a button images without alt texts.

Proposed resolution

We could do something along these lines:

    const announceHandler = () => {
      Drupal.announce();
    };
    editor.commands.get('insertImage').on('execute', announceHandler);
    editor.commands.get('uploadImage').on('execute', announceHandler);

Craft a message that can cater different use cases:

1. Single image uploaded
2. Multiple images uploaded
3. Image inserted (external images)

Image insert vs Image upload can have different announcements. However, what is challenging to distinguish is when multiple images are uploaded at once since all of them are processed asynchronously separately.

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet