* Restore focus highlight on explorer toggle button.
Remove `unset: all` declaration causing `outline`
property to be unset. This allows the default
browser focus highlight to be shown.
* Fix semantics of expandable sections (explorer, toc).
This adds the appropriate aria attributes for the [disclosure pattern](https://www.w3.org/WAI/ARIA/apg/patterns/disclosure/examples/disclosure-image-description/#javascriptandcsssourcecode) and uses `visibility: hidden` to remove the hidden elements from the focus order without disrupting the animations. Further work is needed on the tree view nodes.
* Run prettier for SCSS files.
