feat(sbb-tab-group): add size s variant

[MarioCastigliano]

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Issue

This PR Closes #2682

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• Tests for the changes have been added (for bug fixes / features)
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Changes

Changes in this pull request:

Browsers

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• Bugfix
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Does this introduce a breaking change?

• Yes
• No

Other information

[TomMenga]

LGTM 👍

[mcilurzo]

ux review: https://www.figma.com/design/9r6xSfNmEfCFxl1yFYedrj?node-id=38640-119511#814536681

[jeripeierSBB]

LGTM, thank you!