Close button consistency across components

Created by: ffoodd

Fixes #30917 (closed), fixes #27163 (closed)

Set width & height on .close to:

• have a consistent outline on focus,
• make close button consistent across components: dropping paddings makes all close button the same size, and less dependent about its context.

I'm not convinced about the new $alert-close-offset variables but didn't find a cleaner way yet.

Might be valuable to backport to v4, too.