[v4] add --radius, similar to --spacing

[RebeccaStevens]

Spacing can be customized for all variants with:

@theme {
  --spacing: 4px;
}

It would be great if this could also be done for other things such as radius as well:

@theme {
  --radius: 4px;
}

Currently, all the different radius variants need to be declared:

@theme {
  --radius: 4px;
  --radius-xs: calc(var(--radius) * 0.5);
  --radius-sm: calc(var(--radius) * 1);
  --radius-md: calc(var(--radius) * 1.5);
  --radius-lg: calc(var(--radius) * 2);
  --radius-xl: calc(var(--radius) * 3);
  --radius-2xl: calc(var(--radius) * 4);
  --radius-3xl: calc(var(--radius) * 6);
  --radius-4xl: calc(var(--radius) * 8);
}