Clickable elements of package file browser & copy dropdown aren't accessible

[ravindUwU]

Screenshot of the package file browser taken with tabbing order annotations enabled in the "Accessibility" tab of Firefox devtools:

We should probably add:

1. The breakcrumb, parent directory, directory and show more links aren't keyboard accessible.
   • These are already <a>s, but without hrefs. Specify tabindex="0" explicitly.
2. The copy to clipboard buttons (and therefore the copy dropdown) aren't keyboard accessbile.
   • Button role for the trigger.
   • Listbox roles and popup attributes for the menu & and its items.
   • Key press handlers for the menu items (↑ & ↓ to switch items, Esc to close the menu).
3. The add to collection switch isn't keyboard accessible.
   • Toggle button role for the <label>.

[MartinKolarik]

This would be a nice improvement for sure, would you like to send a PR?