Add CultureInfo.GetParents() and CultureInfo.GetSelfAndParents() extensions

[Copilot]

Adds two new CultureInfo extension methods for traversing the culture hierarchy.

• GetParents() — yields ancestors from immediate parent up to InvariantCulture; returns empty sequence for InvariantCulture itself
• GetSelfAndParents() — yields the culture itself followed by GetParents()

new CultureInfo("en-US").GetParents();
// → [en, InvariantCulture]

new CultureInfo("en-US").GetSelfAndParents();
// → [en-US, en, InvariantCulture]

CultureInfo.InvariantCulture.GetParents();
// → []

[Copilot]

Pull request overview

Adds CultureInfo traversal helpers to PowerKit, enabling consumers to enumerate a culture’s ancestor chain (and optionally include the culture itself) in a consistent, lazy manner.

Changes:

• Added CultureInfo.GetParents() to enumerate parent cultures up to CultureInfo.InvariantCulture (exclusive for invariant itself).
• Added CultureInfo.GetSelfAndParents() to enumerate the culture followed by its parents.
• Added unit tests covering both methods and key cases (en-US, en, and InvariantCulture).

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
PowerKit/Extensions/CultureInfoExtensions.cs	Introduces the two new CultureInfo extension methods for hierarchy traversal.
PowerKit.Tests/CultureInfoExtensionsTests.cs	Adds tests validating expected traversal sequences and invariant behavior.

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