fix(stack): Hide leading, consectuive, and trailing dividers with CSS

[pawelgrimm]

Short description

Previously, if you used <Stack> with the dividers option, and one of the children returned null, we would still display a divider between each child, potentially resulting in leading, consecutive, or trailing dividers. Now, we hide those extra dividers with CSS.

📸 Demo

Before	After

PR Checklist

• Added tests for bugs / new features
  • Covered by existing visual regression tests
• Updated docs (storybooks, readme)
  • Not needed, IMHO
• Reviewed and approved Chromatic visual regression tests in CI

[doistbot]

This PR hides leading, consecutive, and trailing dividers in Stack with CSS, fixing stray dividers when children render null.

Few things worth tightening:

• The styles.stack class is applied unconditionally, so a user-supplied <hr> as a first, last, or adjacent child can get hidden even when dividers="none". Consider applying the class only when dividers are enabled, or scoping the selectors to the generated Divider.
• Text-node children break the sibling-selector logic: in <Stack dividers="primary"><div>A</div>B</Stack>, the needed divider before B gets hidden since CSS selectors only see elements. Wrapping primitive children or marking their dividers would fix this.
• The existing renders dividers when instructed to do so snapshots need updating — they don't expect the new stack class on every rendered Stack, so the Jest suite will fail.

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