feat: tailwind-variants

[sandros94]

Following the work on Nuxt UI v3, migrate to tailwind variants.

Elements

• [ ] dark-mode
• [x] links
• [x] links-group
• [x] headline
• [x] headline-span

Layouts

• [x] card
• [x] footer
• [x] header
• [x] main
• [x] page

Units

• [x] figure
• [ ] hero

[sandros94]

I'm probably going to do this progressively, while creating new components or editing existing ones