Process all nested css rules

[hpohlmeyer]

This change will process all nested rules instead of only the most nested one. This enables support for more complex selectors like the ones used by tailwind v4:

@layer utilities {
  .hover\:text-red-500 {
    @media (hover: hover) {
      &:hover {
          color: var(--color-red-500);
      }
    }
  }
}

The test mock implementation falls too short to properly test this feature. Neither jsdom nor happy-dom have a proper implementation of the behavior the test relies on, so I switched to the vitest browser mode. The switch did not affect any other test.

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Fixes storybookjs/storybook#31410