Add "animated" difference display type

[rparrett]

Just an idea for another display option to complement "side by side", "swipe", "onion skin" etc.

Those are nice, but I often find myself just clicking back and forth between "previous" and "new" so that I can see the whole screenshot. This seems like a useful strategy for understanding differences like subtle lighting changes.

Roughly, something like what this does:

const prev = document.getElementById('reka-tabs-v-8-trigger-previous');
const next = document.getElementById('reka-tabs-v-8-trigger-new');
let toggle = false;

setInterval(() => {
  (toggle ? next : prev)
    .dispatchEvent(new KeyboardEvent('keydown', { key: 'Enter' }));
  toggle = !toggle;
}, 500);