[main_0.34][cherry-pick] Move PlatformProviding code to FluentUI_common

[mischreiber]

Cherry-pick of #2198. Original notes below.

Platforms Impacted

• iOS
• visionOS
• macOS

Description of changes

Despite all the tricky attempts to handle multiple build systems, we're still seeing reports in the wild of our weird Platform[.+]Providing cross-module hooks breaking down. So in the name of cleaner, easier integration, let's just move those into FluentUI_common.

Verification

Verified that themeing works as expected in iOS, macOS, and visionOS.

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

Microsoft Reviewers: Open in CodeFlow