Added Document Export Support

[fhasse95]

This PR adds document export support to SkipKit. This is achieved by a new withDocumentExporter view modifier, which presents the native export flow on both platforms. On iOS, it uses SwiftUI's fileExporter. On Android, it uses ACTION_CREATE_DOCUMENT and copies the source file to the user-selected destination.

For more information, check out the updated README file.

A complete example is also available in the DocumentPickerPlayground of the Skip Showcase app.

Related PR:

• Added Document Export Support skipapp-showcase#119

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• REQUIRED: I have signed the Contributor Agreement
• REQUIRED: I have tested my change locally with swift test
• OPTIONAL: I have tested my change on an iOS simulator or device
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• AI was used to generate or assist with generating this PR. Please specify below how you used AI to help you, and what steps you have taken to manually verify the changes.

The initial implementation was generated with assistance from Codex. The resulting code was manually reviewed, refined where necessary, and validated using both the Skip Showcase app and my own app on iOS, iPadOS, macOS (Catalyst), and Android to ensure correctness and consistent cross-platform behavior.

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