Add ForEach view for iterating over collections

[csjones]

⚡ Summary

This PR introduces a new ForEachView component to Slipstream, enabling iteration over collections to dynamically render repeated content. It is inspired by SwiftUI’s ForEach, but tailored for Slipstream’s ViewBuilder system. For example:

ForEach(articles, id: \.id) { article in
    ArticleView(article: article)
}

The implementation provides type safety through generics and RandomAccessCollection constraints, and streamlines the rendering of list-based UI components by supporting both Identifiable elements and explicit id key paths.

Unit tests cover:
• Iteration with Identifiable collections
• Iteration with custom id key paths
• Rendering links and styled elements via modifiers
• Handling empty collections with explicit fallback content

☑️ Checklist

• Checked locally
• Add tests
• Add documentation