Add SwiftUI PillButton

[laminesm]

Platforms Impacted

• [x] iOS
• [x] visionOS
• [ ] macOS

Description of changes

This PR implements a new SwiftUI PillButtonView.

• The new SwiftUI pill button adds support for a leading image icon as defined in the Fluent 2 designs.
• The PillButtonView uses the existing PillButtonTokenSet and adds the necessary tokens for the leading icon and the unread dot offset values.
• The new pill button uses a PillButtonViewStyle which is used to configure the button based on its style and tokens.
• The PillButtonView is initialized with a PillButtonViewModel which is the observable class used to track the unread dot and leading image states.

Binary change

TBD

Verification

• Visual verification on iPhone and Vision Pro.
• Used the accessibility inspector to VoiceOver, accessibility labels and traits.

Visual Verification

iPhone

Standard	Unread dot	Icon	Icon + unread dot

Action	Large text	Focus ring

Vision Pro

Standard	Unread dot	Icon	Icon + unread dot

Pull request checklist

This PR has considered:

• [x] Light and Dark appearances
• [ ] iOS supported versions (all major versions greater than or equal current target deployment version)
• [x] VoiceOver and Keyboard Accessibility
• [x] Internationalization and Right to Left layouts
• [ ] Different resolutions (1x, 2x, 3x)
• [x] Size classes and window sizes (iPhone vs iPad, notched devices, multitasking, different window sizes, etc)
• [ ] iPad Pointer interaction
• [x] SwiftUI consumption (validation or new demo scenarios needed)
• [ ] Objective-C exposure (provide it only if needed)

Microsoft Reviewers: Open in CodeFlow