Add 'end of video' announcement to alert screen readers when a video has finished

[kirsty-hames]

Subject of the enhancement

To improve user experience, add a screen reader alert to inform learners when a video has finished playing. Currently the silence is the only audio indicator which may be perceived as content not loading.

This could be implemented by triggering a live region when the video is finished.

[oliverfoster]

adaptlearning/adapt-contrib-core#673

When this goes through you'll have

import notify from 'core/js/notify';

await notify.read('end of video');