fix: Remove parenthesis around sole list items

[RedGuy12]

Fixes #3545, but it affects more places as well.

Description

I added this to the hug_parens_with_braces_and_square_brackets unstable style since there was a lot of overlapping code. I renamed it to concise_nested_brackets to reflect this. That is a breaking change if anyone was using --enable-unstable-feature hug_parens_with_braces_and_square_brackets, but to my knowlege, the existence of certain unstale features is not gurenteed to be stable. I can add the old name back as a (deprecated) alias or revert the rename altogether if so desired.

This does introduce more places where #4036 would be triggered:

items = [  # type: ignore
    (  # type: ignore
        {"key1": "val1", "key2": "val2", "key3": "val3"}
        if some_var == "longstring"
        else { "key": "val" }
    )
]

Checklist - did you ...

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• [y] Add new / update outdated documentation?

[github-actions]

diff-shades reports zero changes comparing this PR (cbf5730) to main (2f88085).

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[JelleZijlstra]

Suggested change

	Parenthesises around sole list items will also be removed for similar reasons. Trailing
	Parentheses around sole list items will also be removed for similar reasons. Trailing

[JelleZijlstra]

Thanks, these changes look good.

However, I think I'd prefer to keep feature names stable through the year. The main motivation for adding --enable-unstable-feature was to allow users who want to keep some feature on to just turn on the feature in their config and have it keep working.

Also, I don't want to accept a PR that directly adds a new unstable style. I see the unstable style as a holding pen for style changes that we'd originally put into preview but later ran into issues with. New style features should go into preview instead.