Storybook: Add UnitControl story

[im3dabasia]

Part of #67165

What?

This PR will add stories for UnitControl component in the Storybook.

Why?

As part of the ongoing effort to improve component documentation and testing (tracked in #22891), we need comprehensive stories for all Block Editor components.

Testing Instructions

1. Run npm run storybook:dev
2. Open the storybook on http://localhost:50240/
3. Check the UnitControl stories.

Screenshots or screencast

Screen.Recording.2024-11-27.at.4.57.25.PM.mov

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