chore: Create a story for UnitInput component

[devin-ai-integration]

Description

This PR introduces Storybook stories for the UnitInput component to showcase its different states and behaviors. It is essential for visual testing and ensures that the component's functionality is maintained across changes.

Related issues (if any)

Fixes: https://github.com/MetaMask/metamask-extension/issues/

Manual testing steps

• Go to the latest build of storybook in this PR
• Navigate to the UnitInput component in the Components/ folder.

Screenshots/Recordings

Pre-merge author checklist

• I’ve followed MetaMask Coding Standards.
• I've completed the PR template to the best of my ability
• I’ve included tests if applicable
• I’ve documented my code using JSDoc format if applicable
• I’ve applied the right labels on the PR (see labeling guidelines). Not required for external contributors.

Pre-merge reviewer checklist

• I've manually tested the PR (e.g. pull and build branch, run the app, test code being changed).
• I confirm that this PR addresses all acceptance criteria described in the ticket it closes and includes the necessary testing evidence such as recordings and or screenshots.

[github-actions]

This PR has been automatically marked as stale because it has not had recent activity in the last 60 days. It will be closed in 14 days. Thank you for your contributions.