Support key param for sortBy and sortByDescending

[iliagon]

Summary

This PR adds support for sorting by key directly on the Sequence and AsyncSequence types, similar to PR winterbe#33 from the original sequency repository.

Changes

• Added key parameter support to sortedBy and sortedByDescending methods for both sync and async sequences
• Methods now accept either a selector function or a key string: .sortedBy("a") or .sortedBy(it => it.a)

Example Usage

// Using key string
sequenceOf({a: 4}, {a: 1}, {a: 3})
    .sortedBy("a")  // ✅ Now works!
    .sortedByDescending("a")  // ✅ Now works!

// Using selector function (still works)
sequenceOf({a: 4}, {a: 1}, {a: 3})
    .sortedBy(it => it.a)
    .sortedByDescending(it => it.a)

Based on PR #33 from the original sequency repository.