Add support for changing the slider value using up and down also

[esseb]

Call event.preventDefault() to prevent unwanted scrolling.

[esseb]

https://developer.mozilla.org/en-US/docs/Web/Accessibility/ARIA/ARIA_Techniques/Using_the_slider_role recommends supporting Up and Down also to interact with the slider, and is also how <input type="range"> works in most browsers today.

I also call event.preventDefault() to prevent unwanted scrolling, which is more likely to happen when the user presses up or down, but could also happen with left and right on a wide page.

[davidchin]

Thanks for your work @esseb!