Move vision token file to FluentUI_common module

[joannaquu]

Platforms Impacted

• iOS
• visionOS
• macOS

Description of changes

Although theoretically we only need the vision token file in the iOS module, we still reference it in the common module in FluentTheme.swift. Moving it to FluentUI_common.

Verification

Demo app builds

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

[mischreiber]

We can take this change to unblock the build for right now, but I'm working on a new way to inject platform-specific tokens. It's intended for macOS, but visionOS will benefit as well.