Move hug_parens_with_braces_and_square_brackets into the unstable style

[JelleZijlstra]

Primarily because of #4036 (a crash) but also because of the feedback
in #4098 and #4099.

Description

Checklist - did you ...

• Add an entry in CHANGES.md if necessary?
• Add / update tests if necessary?
• Add new / update outdated documentation?

[github-actions]

diff-shades results comparing this PR (8fe1ea5) to main (d6e11ca). The full diff is available in the logs under the "Generate HTML diff report" step.

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│ 18 projects & 1062 files changed / 66 767 changes [+37 478/-29 289] │
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│ ... out of 2 532 850 lines, 12 161 files & 22 projects              │
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Differences found.

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