feat: forbid redundant trailing colon in slice (fixes #1071)

[f1sherFM]

I have made things!

Added WPS367 (RedundantTrailingSliceViolation) to detect redundant trailing colons inside subscript slices that the AST parser cannot distinguish. Introduced RedundantTrailingSliceVisitor in visitors/tokenize/subscripts.py to catch cases like a[1:4:], a[1::], and a[:4:] at the token level. Also extended the existing RedundantSubscriptViolation test suite with previously uncovered step=1 scenarios (::1, 1::1, 0::1).

Checklist

• I have double checked that there are no unrelated changes in this pull request (old patches, accidental config files, etc)
• I have created at least one test case for the changes I have made
• I have updated the documentation for the changes I have made
• I have added my changes to the CHANGELOG.md

Related issues

Closes #1071

🙏 Please, if you or your company is finding wemake-python-styleguide valuable, help us sustain the project by sponsoring it transparently on https://opencollective.com/wemake-python-styleguide. As a thank you, your profile/company logo will be added to our main README which receives hundreds of unique visitors per day.

[sobolevn]

Please create a structure for this. Even a tuple with a type alias will work. list[list] is not correct.

[sobolevn]

if you need a mutable structure you can use TypedDict

[f1sherFM]

@sobolevn
Fixed both points:

1. Changed .. versionadded:: 1.6.3 → 1.7.0 — you're right, this is a new feature.

2. Replaced list[list] with a proper TypedDict (_SliceBracketState) for bracket tracking. Since we need mutability during token traversal, I used TypedDict with module-level string constants for field names to avoid WPS226. Pushed as amended commit.