BoxControl: Refactor and unify the different sides implementation

[youknowriad]

Related #67625

What?

In order to implement #67625, I need first to simplify the code of the BoxControl component. It contains three duplicated subcomponents that can be easily merged in one by adding an extra side prop.

This PR does that.

Testing Instructions

There should be no functional or visual change in this PR. You can test this PR in storybook (the different BoxControl stories) and ensure they look and behave exactly like trunk.

[github-actions]

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Co-authored-by: youknowriad <[email protected]>
Co-authored-by: tyxla <[email protected]>

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[tyxla]

Thanks for the great unification work @youknowriad 🙌

Did some testing, and according to the stories, there might be a few regressions. Please take a look at the individual comments.

[github-actions]

Flaky tests detected in df9490d.
Some tests passed with failed attempts. The failures may not be related to this commit but are still reported for visibility. See the documentation for more information.

🔍 Workflow run URL: https://github.com/WordPress/gutenberg/actions/runs/12199987455
📝 Reported issues:

• [Flaky Test] submenu click on the arrow interactions #61884 in /test/e2e/specs/editor/blocks/navigation-frontend-interactivity.spec.js

[tyxla]

Here's another edge case that looks broken:

• Open http://localhost:50240/?path=/story/components-boxcontrol--default
• Unlink
• Input different values in each side control
• Choose a different unit (em for example) for at least one of the sides
• Link again.

On trunk: You'll see Mixed without the unit:

In this branch: You'll see Mixed but with a px unit, even if some of the units are em:

Note that even if I select em unit for all sides, I still see px after linking them back.

[youknowriad]

I think the best path is to consider "mixed units" as "mixed value" and disable the unit like in trunk. It should be done now.

[tyxla]

Code looks good, and this tests well, with no visible changes. Amazing unification work @youknowriad 🚀

Left a few last nits, feel free to ship afterwards.

Thank you 🙌

[youknowriad]

Thanks for the review Marin