[Shimmer] Introduce ShimmerLineConfiguration to enable more control over ShimmerLineView

[Des Marks (nodes11)]

Platforms Impacted

• iOS
• visionOS
• macOS

Description of changes

The existing ShimmerLinesView implementation does not allow for a Shimmer view that has lines with varying widths and heights. To enable this, I have created a ShimmerLineConfiguration class that defines the characteristics of a line and updated the ShimmerLinesView to take an optional array of ShimmerLineConfiguration objects that make up the shimmer view.

Binary change

(how is our binary size impacted -- see https://github.com/microsoft/fluentui-apple/wiki/Size-Comparison)

Verification

Validated in demo controller. Validated in internal use case.

Visual Verification

Simulator.Screen.Recording.-.iPhone.17.Pro.-.2026-08-04.at.13.46.34.mov

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