Add accessibility support

[davidacm]

Hi there. I am visually impaired and this extension is very useful for me, I made accessibility improvements in the past, before the code was completely rewritten. I never made a pull request because I wanted to test first.
This extension helped me a lot to improve my listening in new languages. Listening is a difficult skill to acquire for a blind person, because we don't see the context nor can we read subtitles in real time and the voices of the actors at the same time.

In order to help visually impaired people, I wanted to propose implementing some accessibility improvements, in particular so that blind people can use this extension with screen readers and braille displays, I can implement the accessibility features.

The improvements I think for now are the following:

• Add semantic structure for the subtitle widget. So, blind people will be able to move the focus to the subtitle at any time.
• Automatic reading of subtitles using voice and braille. Possible with aria attributes, and the user must be able to toggle this.
• manual reading of current subtitle using voice and braille. A keyboard command should be assigned for this.
• correct labeling and roles of UI widgets.
• Playback speed adjustment via keyboard commands.
• Return to normal playback speed.

These features do not require changes to the extension's existing functionality.
But I would like to suggest some additional changes that would change the existing functionality, it's okay if it can't be implemented for some reason.
I will write them in order of priorities:

1. Keyboard commands: arrows are usually used to go back, forward, or control the volume on streaming platforms. This feature is useful for people who only use the keyboard and not the mouse. By overwriting these commands, those of us who require such functionalities lose the ability to advance by time or control the volume. One possibility for a softer change would be to implement 2 keyboard layouts.
2. Subtitle progress bar. Currently it's not fully accessible, but it would be useful to have a list of subtitles that can be navigated with arrows, and to be able to navigate to the subtitle that we choose. If you have used Microsoft Teams, it has a similar widget for recordings. I think it would not be simple to implement, but I can do it after having covered the most important features.
3. Custom keyboard commands: I've seen it in some extensions so the possibility exists, but for now I don't have any idea of the best way to implement it. I think it would require some internal changes to the code, perhaps some extra library for keyboard handling, and a configuration view to assign the gestures.

Thank you in advance,I hope that together we can develop the improvements necessary to launch the first extension for language learning accessible for visually impaired people.

Best regards,
David