UIKitBackend: Implement slider for tvOS

[stackotter]

UIKit doesn't support sliders on tvOS. But tvOS's own settings app has plenty of sliders, so I think it'd be reasonable for us to provide a slider implementation for tvOS. We'll likely have to make some custom sort of UIView and get it to match as close as we can. Making a slider ourselves isn't ideal cause then we need to update the implementation each time Apple introduces a new design language (such as liquid glass), but I think it's the best we can do without Apple providing a first-party solution.