[Fluent 2 iOS] Separator color update

[laminesm]

Platforms Impacted

• iOS
• macOS

Description of changes

The Separator has been updated to match its fluent 2 design.
Design requested the shadow style be removed since it's unused in fluent 2. The affected controls are TabBar, SideTabBar and ShyHeaderView. They will now be using the default separator.
Following the previous discussions on how to deal with objc colors, the Colors extension was removed and I created a static objc function which takes in a FluentTheme and returns the right token. As a consequence in PopupMenuController, I called FluentTheme.shared since I couldn't access the FluentTheme from a view.

Verification

(how the change was tested, including both manual and automated tests)

Before	After

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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