BorderControl: Fix vertical alignment of inner slider control

[aaronrobertshaw]

What?

Improve the styling of the BorderControl to ensure the inner slider control is more reliably vertically aligned to the center out of the box.

Why?

This change is being made to the component itself so there is less that any consumer may need to tweak for everything to be neatly aligned in their UI.

How?

Adds back the display: flex rule that the component originally used to solve this alignment (#37769 (comment))

Testing Instructions

1. Checkout this branch and start up the Storybook
   • npm run storybook:dev
2. Confirm the slider within the BorderControl is still vertically aligned to the center.
3. Check the alignment for the BorderBoxControl's linked view.
4. If you're still not convinced. You can apply the changes from this branch onto #37770 and confirm the alignment now works in the block editor
   • Create a post, add a group block, and select it
   • Take a look at the border control in the inspector controls sidebar

Screenshots or screencast

Screenshots below are from #37770 and the block editor.

Before	After

[andrewserong]

LGTM!

✅ Confirmed that in the Storybook version of the component there's no apparent difference
✅ Confirmed that in real-world usage in #37770 this is required to switch the element from being inline-flex to being flex to vertically centre the component in the available space

Sounds like a good place to put it to me, to ensure that the slider in this context is positioned correctly 👍

Before	After