lib.lists: add uniqueByString

[roberth]

Adds a new function to remove duplicates based on a string key function. This provides stable deduplication that preserves input order, unlike uniqueStrings, which doesn't preserve order, and allows custom key extraction unlike unique which compares elements directly.

Also adds docs cross-references between unique, uniqueStrings, and uniqueByString to help users discover the appropriate function for their use case.

Context

• First written for Infer devShell deps, provide smaller one nix#14489
  • deduplicating a large number of build inputs gathered from multiple derivations, so that the resulting devShell is reviewable, and doesn't waste the finite env+args space in processes.

Things done

• Built on platform:
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• Tested, as applicable:
  • NixOS tests in nixos/tests.
  • Package tests at passthru.tests.
  • Tests in lib/tests or pkgs/test for functions and "core" functionality.
• Ran nixpkgs-review on this PR. See nixpkgs-review usage.
• Tested basic functionality of all binary files, usually in ./result/bin/.
• Nixpkgs Release Notes
  • Package update: when the change is major or breaking.
• NixOS Release Notes
  • Module addition: when adding a new NixOS module.
  • Module update: when the change is significant.
• Fits CONTRIBUTING.md, pkgs/README.md, maintainers/README.md and other READMEs.

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[hsjobeki]

Motivation for #452088
Linked lists would allow to cut down time complexity from O(n^2) to O(n log n).
Without linked lists this has the same behavior and time complexity as unique.