Rename --aspect-ratio-* theme key to --aspect-*

[reinink]

This PR renames the --aspect-ratio theme key to --aspect. This is to match what we've done with other theme keys where they match the utility names, like --ease and --leading.

  @theme {
-   --aspect-ratio-retro: 4 / 3;
+   --aspect-retro: 4 / 3;
  }

Additionally, I've also converted the existing aspect-video static utility to a theme value. This will allow people to override this utility in their own projects—something that's not possible with static utilities. This change feels appropriate since the video aspect ratio is subjective, unlike other static utilities like aspect-square.

@theme {
  --aspect-video: 4 / 3; /* N64 baby! */
}