Fix crash with #available and .environment on visionOS

[joannaquu]

Platforms Impacted

• iOS
• visionOS
• macOS

Description of changes

There seems to be an apple bug where using #available with .environment causes a crash on visionOS. Potentially related to #1984. As a workaround, move the modifier to its own ifdef.

While I'm here, replace magic numbers with global tokens.

Binary change

Total increase: 1,240 bytes
Total decrease: 0 bytes

File	Before	After	Delta
Total	31,099,792 bytes	31,101,032 bytes	⚠️ 1,240 bytes

Full breakdown

File	Before	After	Delta
FluentList.o	72,352 bytes	73,400 bytes	⚠️ 1,048 bytes
__.SYMDEF	4,811,616 bytes	4,811,808 bytes	⚠️ 192 bytes

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