Refactor `repeated_keys()` to use `ComparableExpr` (#5696)
## Summary
Replaces `DictionaryKey` enum with the more general `ComparableExpr`
when checking for duplicate keys
## Test Plan
Added test fixture from issue. Can potentially be expanded further
depending on what exactly we want to flag (e.g. do we also want to check
for unhashable types?) and which `ComparableExpr::XYZ` types we consider
literals.
## Issue link
Closes: https://github.com/astral-sh/ruff/issues/5691
