[macOS] Initial implementation of message bar for macOS

[mischreiber]

Platforms Impacted

• iOS
• visionOS
• macOS

Description of changes

Creating a new SwiftUI MessageBar component for macOS.

Additionally, this also creates a MessageBarStack for managing multiple message bars at a time. This comes with an AppKit wrapper, MessageBarStackHostingView, for hosting in an NSView-based context.

Finally, this reorganizes a few helper files into the Common library.

Verification

Multiple demo apps, testing both SwiftUI and AppKit versions.

Visual Verification

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