Carousel improvements

[ffoodd]

Carousels might be improved, or at least it worths the try:

• WAI ARIA carousel best practices
• WAI Carousel Tutorial

Required

• [x] Add a play/pause button
• [ ] Enhance tab order (especially when a carousel item contains a focusable element)
• [ ] Focus the selected item
• [x] ~~Disable prev/next controls when not going to wrap~~ (#573)

Nice to have:

• [x] ~~Progressive enhancement: adding prev/next buttons through JS, mostly~~
• [ ] Announce the current item using live region
• [x] ~~Pause on hover~~ → already done, as long as carousel autoplays.
• [ ] Hide transitioning elements from ATs

Docs:

• [ ] Add link to WAI Tutorial, maybe? https://www.w3.org/WAI/tutorials/carousels/
• [ ] Interesting considerations in this WebAIM mailing list

x-ref Bootstrap #31908