site: make REPL user menu keyboard accessible

[geoffrich]

Closes sveltejs/sites#494

The current user menu in the REPL wasn't keyboard- or AT- accessible since it only activated on hover. I refactored it to activate on click/button trigger instead. Clicking outside or moving focus outside the menu auto-closes it.

Briefly toyed around with also activating it on hover, but that made things too confusing. I think the chevron icon is a clear enough indication that this is interactable (and on mobile you had to tap it anyway).

Some of the icons increase opacity on hover - I updated things to increase opacity on focus as well.

Also refactored some of the styles to use flexbox, since that made things cleaner.

Since this PR bumps site-kit, it also includes a few changes from the latest release (removing the background blur on the search overlay and fixing the double-underline on nav links)

Screenshots

Desktop

Mobile

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