`Visibility` instances

[nsaunders]

See https://github.com/sebastiaanvisser/clay/blob/master/src/Clay/Display.hs#L189

• Visibility should not have an Auto instance because auto is not a valid value for the visibility property
  • MDN Web Docs: visibility
  • 
• Visibility should have an Unset instance
  • MDN Web Docs: visibility

[turion]

Makes sense, PR welcome :)