Make submenus visible during keyboard navigation

I've noticed that the site didn't make submenus visible during keyboard navigation, making everything less intuitive.

This change shows the submenu when it has focus on it's `button` element or focus within itself.

I'm not sure if the intention behind this would be a "toggle" `button` (with it's corresponding ARIA states) instead of this, as the button currently gains focus but doesn't trigger anything on click.

Author: royeden

src/components/Navbar.module.css

@@ -141,7 +141,9 @@
   }
 }
 
-.item:hover .submenu {
+.item:hover .submenu,
+.item:focus-visible .submenu,
+.submenu:focus-within {
   opacity: 1;
   max-height: 400px;
 }