sl-carousel, sl-carousel-item: accessibility updates

[doctor-spaceman]

Hello! I'd like to suggest the following accessibility changes for the carousel component.

When pagination is enabled:

• Add role="tabpanel" to each sl-carousel-item.
• Add an id to each sl-carousel-item that can be referenced by the aria-controls attribute on each tab.
• Remove aria-controls from the pagination part, adding it to each tab element instead, and populating it with the corresponding id of each tabpanel .
• Add an id to each tab.
• Add aria-labelledby to each sl-carousel-item and populate it with the id of the corresponding tab.

I'm willing to try creating a PR for this.

References:

• MDN: tab role
• MDN: tab role example
• MDN: tab panel role
• MDN: tablist role
• MDN: aria-controls