Material Carousel a11y improvements

[StephDriver]

From #4194 Sample 5.2 on a Material theme homepage:

Carousel not identified as such. Navigating by heading gets to visible item on carousel, as an H2. But as this is a carousel, this keeps changing making it awkward when navigating past it, then back to find previous heading has changed.

[joemull]

We think we should move all themes to an accessible, ES6-based carousel tool like glide.js, which we used for Hourglass.

[StephDriver]

When updating this, consider WCAG 2.1 Keyboard Accessible and WCAG 2.2 Enough Time. Neither are currently supported by the carousel.

[StephDriver]

adding that this can also be a particular annoyance when there is only one image - as the carousel still moves, and the page background shows for a split second. If the image is has a lot of black background, and the page is white, this makes it 'flash' between the two extremes. The carousel doesn't move fast enough for this to contravene the 3 flash in a second threshold, but it does move swiftly enough for this to be visually uncomfortable.