Platforms Impacted

• iOS
• visionOS
• macOS

Description of changes

This moves the duplicate implementations of FluentThemeable for NSView and UIView into a shared file in FluentUI_common.

No more duplcation, and also preemptively solves a problem with Swift 6 that would require @retroactive.

Verification

No functionality changes. Code builds and demos run as expected on all platforms.

Pull request checklist

This PR has considered:

• Light and Dark appearances
• iOS supported versions (all major versions greater than or equal current target deployment version)
• VoiceOver and Keyboard Accessibility
• Internationalization and Right to Left layouts
• Different resolutions (1x, 2x, 3x)
• Size classes and window sizes (iPhone vs iPad, notched devices, multitasking, different window sizes, etc)
• iPad Pointer interaction
• SwiftUI consumption (validation or new demo scenarios needed)
• Objective-C exposure (provide it only if needed)

Microsoft Reviewers: Open in CodeFlow