Add uniqued(on:uniquingWith:) and uniqued(uniquingWith:) conflict-resolving overloads

[shubhransh-gupta]

Summary

Resolves #188.

Currently, Sequence.uniqued(on:) only keeps the first occurrence for duplicate keys and drops subsequent elements without allowing custom conflict resolution. This PR adds conflict-resolving overloads to Sequence:

• uniqued(on:uniquingWith:)
• uniqued(uniquingWith:) (for Sequence where Element: Hashable)

These methods maintain the existing Dictionary(_:uniquingKeysWith:) and keyed(by:resolvingConflictsWith:) semantics by calling combine with the current accumulated element and the newly encountered element, preserving the relative index positioning of the first occurrence.

Changes

• Added uniqued(on:uniquingWith:) and uniqued(uniquingWith:) in Sources/Algorithms/Unique.swift.
• Updated documentation and API specifications in Guides/Unique.md.
• Added unit tests in Tests/SwiftAlgorithmsTests/UniqueTests.swift covering collision resolution, order preservation, empty sequences, and throwing closures.

Checklist

• Code style follows the rest of the project
• Unit tests added and passing (226 tests passed)
• Documentation updated in Guides/Unique.md

[kateway0694-lab]

@caitlinbrahler