feat(react): add unstable_Stack component

[joshblack]

Closes #9821

This PR adds in a Stack component to be used for vertical spacing along with other components in the design system. This is a useful utility in a component-based model when margin is no longer used by components.

By default, the Stack uses a gap prop where you can provide a number or a string. If it's a number, it will map to our spacing scale. If it's a string, it will be a custom value and requires a unit.

Changelog

New

• Add Stack component, related stories, and tests

Changed

• Update storybook to include Stack in carbon-react

Removed

Testing / Reviewing

• Verify tests pass as expected
• Verify CSS is included as expected
• Verify story renders as expected
• Look over the API and make sure it meets our API requirements / is understandable

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[joshblack]

Question for you @tay1orjones, my code had step as the prop but could easily switch to spacing if people like that better

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[tay1orjones]

I'm super into this implementation! I think the usage of grid gap is a fantastic idea.

I'm happy to knock out some tests for it once any other feedback is incorporated

[tay1orjones]

Newest changes feel great! 🔥 Consolidating down to just gap and also very much in support of the VStack and HStack exports

[dakahn]

Nice work!