[macOS] Use new Stroke Alpha tokens for message bar separators

[mischreiber]

Platforms Impacted

• iOS
• visionOS
• macOS

Description of changes

• Adding a new pair of color tokens, .strokeAlpha1 and .strokeAlpha2 from the design kit.
• Using the first of these for MessageBar.

Verification

(how the change was tested, including both manual and automated tests)

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