Reintroduce deprecated theme values

[adamwathan]

This PR reintroduces the blur, shadow, drop-shadow, and rounded utilities that were removed in #14849, just to preserve backward compatibility as much as possible.

These values are still considered deprecated, and we register them as inline reference with the theme to ensure they don't produce any CSS variables in output:

/* Deprecated */
@theme default inline reference {
  --blur: 8px;
  --shadow: 0 1px 3px 0 rgb(0 0 0 / 0.1), 0 1px 2px -1px rgb(0 0 0 / 0.1);
  --drop-shadow: 0 1px 2px rgb(0 0 0 / 0.1), 0 1px 1px rgb(0 0 0 / 0.06);
  --radius: 0.25rem;
}

These values won't be included in the documentation, and in the future we'll add an option to explicitly register things as deprecated so that they don't appear as completions in IntelliSense either.