Separator for table direct children

[ffoodd]

Fixes #34732, closes #34764 as a replacement, basically refactoring the evolution from v4 instead of adding old code.

The main culprit here is getting back to a 2px border, since for whatever reason a 1px border is hidden behind cells borders. As pointed out by @adityadees, a 1.1 multiplicator would be enough, but feels like a magic number to me.

IMHO @mdo we'd better recommend to use our border utilities (eg. .border-dark in the initial issue, as in this dedicated CodePen) rather than fixing this. Any thought?

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Demo on CodePen.

[mdo]

I feel like the new code is actually simpler somehow? I don't mind it as the path forward. Utilities could work, but ideally stuff like this works out of the box IMO. Maybe I made tables too opinionated in v5 😬

[ffoodd]

So what about this 😄? Do we stick with the 2px border as in this PR, or do we move things again to support this?

[adityadees]

I feel like the new code is actually simpler somehow? I don't mind it as the path forward. Utilities could work, but ideally stuff like this works out of the box IMO. Maybe I made tables too opinionated in v5 😬

Nah, I think since v5 you reduce 2 to 1. But I don't know why?
That's why I just put 1.1 just to add second border bigger than first border without change initial value from v5.

[mdo]

@ffoodd I think we stick with the 2px still. Let's ship it!