UnitControl: Add __unstableInputWidth to prop types

[aaronrobertshaw]

Related: #37769

Description

The UnitControl passes through props to an underlying InputControl component. The InputControl accepts an __unstabledInputWidth prop while the UnitControl types omit this.

This PR adds __unstableInputWidth to the UnitControlProps type.

Testing Instructions

1. Confirm the __unstableInputWidth prop's type for the UnitControl matches the InputControl's.
2. Check there are no errors when running npm run build:package-types
3. Test that the UnitControl continues to function in the Storybook: npm run storybook:dev

Types of changes

Bug fix.

Checklist:

• My code is tested.
• My code follows the WordPress code style.
• My code follows the accessibility standards.
• I've tested my changes with keyboard and screen readers.
• My code has proper inline documentation.
• I've included developer documentation if appropriate.
• I've updated all React Native files affected by any refactorings/renamings in this PR (please manually search all *.native.js files for terms that need renaming or removal).
• I've updated related schemas if appropriate.

[andrewserong]

Good catch, thanks @aaronrobertshaw! This is a property we need exposed on the UnitControl component, so adding it in here makes good sense to me 👍

✅ Confirmed the __unstableInputWidth prop's type for the UnitControl matches the InputControl's.
✅ Checked there are no errors when running npm run build:package-types
✅ Tested that the UnitControl continues to function in the Storybook: npm run storybook:dev and in real world usage in the editor (e.g. padding / cover block min height)

LGTM!

[aaronrobertshaw]

Changelog for this will be added via: #38469