Add `tabindex="0"` by default

[zachleat]

via #48 and more specifically this comment by @chrisbrownie55 #22 (comment)

[zachleat]

Hmm, if the region is not scrollable—is tabindex desirable? Not sure if this should be a default change…

[andria-dev]

@zachleat I think it'd be more desirable than making all code blocks with scrollbars inaccessible to keyboard users and others that need to scroll without a pointer device. A possible alternative would be to use a ResizeObserver on code blocks and determine if a scrollbar is present by checking if element.scrollHeight > element.clientHeight and then apply tabindex="0" or remove it.

I think the most accessible solution would probably be a progressive enhancement where we start with tabindex="0" by default and then include a script that'll do the above ResizeObserver technique to remove or add tabindex="0" when a code block changes shape.