Fix Mac Catalyst build

[thomasjross]

Platforms Impacted

• iOS
• visionOS
• macOS

Description of changes

After some of the changes in the 0.34.0 release to move FluentTheme into the common layer, it's no longer possible to build FluentUI for Mac Catalyst. The changes are relatively simple to get things building again.

Add a target environment check to NSColor+Extensions

• Simply checking if we can import AppKit is not sufficient since the framework can be imported, but NSColor is unavailable on Catalyst.

Use FluentFontInfo type alias

• Since Catalyst has access to ApplicationServices.FontInfo, the reference to FontInfo in the iOS target is ambiguous. Using the type alias resolves this ambiguity.

Binary change

N/A, should have no impact on iOS binary size.

Verification

Swift package for FluentUI was built for iOS, visionOS, macOS, and macOS (Catalyst). I confirmed that the package builds properly for all supported platforms listed in the package definition.

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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