Add stacked topic tag group modifier

[anselmbradford]

When topic tags appear in a sidebar they are stacked, so this PR adds a modifier to stack them at non-jumplink sizes.

Additions

• Add stacked topic tag group modifier

Testing

1. Visit https://deploy-preview-2042--cfpb-design-system.netlify.app/design-system/components/tags#topic-tag-stacked and see there is a stacked topic tag group, which stays stacked at all screen sizes.

[netlify]

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