Add a native macOS icon with Dark Mode and Liquid Glass support

Ladybird's current macOS app icon is a static image and doesn't take advantage of the new icon system. Using Icon Composer, we could have automatic Light/Dark mode variants and Liquid Glass rendering (Clear, Tinted, and Default appearance modes).

Right now, Ladybird's icon doesn't adapt to these system appearance settings and looks out of place next to icons from apps that have already adopted the new format.