Add fluentGlassEffect modifier

[joannaquu]

Platforms Impacted

• iOS
• visionOS
• macOS

Description of changes

glassEffect is unavailable on OSs < 26 and vision. Add a convenience modifier fluentGlassEffect. On older OSes and unsupported platforms, apply a background with .regularMaterial and shadow08.

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)

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