Add CSS rule for [hidden]
Add CSS rule for [hidden]

Effectively the same change as https://github.com/BrightspaceUI/loading-spinner/pull/26, applying the `hidden` attribute to a `d2l-icon` won't actually hide it, turns out. This is because the `display: none` part of `hidden` ends up getting overridden by the `:host { display }` rule defined within `d2l-icon`.
Brightspace/valence-ui-icons