Add Windows Support to SelectableLabel Control

[Copilot]

This PR adds comprehensive Windows support to the Indiko.Maui.Controls.SelectableLabel library, enabling the control to work seamlessly across all major MAUI platforms.

What's Added

Windows Platform Handler

• Implemented a complete Windows handler using WinUI 3's RichTextBlock control
• Enables text selection with IsTextSelectionEnabled = true
• Provides full feature parity with existing Android, iOS, and MacCatalyst implementations

Target Framework Support

• Added net8.0-windows10.0.19041.0 target framework to the library
• Updated sample project to include Windows target for testing
• Set minimum Windows version to 10.0.17763.0 (Windows 10 version 1809)

Complete Feature Implementation

The Windows handler supports all existing SelectableLabel features:

• Text Properties: Text, TextColor, FontAttributes, FontSize, FontFamily
• Layout: LineBreakMode, LineHeight, MaxLines, text alignment
• Formatting: TextDecorations (underline, strikethrough), TextTransform, CharacterSpacing
• Rich Text: FormattedText with multiple spans and individual styling
• Background: BackgroundColor support

Platform-Specific Optimizations

• Uses RichTextBlock for optimal text selection experience on Windows
• Properly maps MAUI properties to WinUI equivalents (e.g., TextAlignment, TextWrapping)
• Handles character spacing conversion from em units to WinUI format
• Implements text truncation using TextTrimming for ellipsis behavior

Bug Fixes

Also fixed existing issues in the core SelectableLabel class:

• Corrected MaxLinesProperty BindableProperty declaration (was incorrectly using nameof(LineHeight))
• Fixed TextTransformProperty BindableProperty declaration (was incorrectly using nameof(TextDecorations))

Developer Experience

• No breaking changes - existing XAML and C# code works unchanged
• Automatic platform detection through conditional compilation
• Consistent API across all platforms
• Updated documentation to reflect Windows support

Usage Example

<TargetFrameworks>net8.0-android;net8.0-ios;net8.0-windows10.0.19041.0</TargetFrameworks>

builder.UseSelectableLabel(); // Automatically registers Windows handler

<idk:SelectableLabel
    Text="This text is now selectable on Windows too!"
    TextColor="Blue"
    FontSize="18" />

The implementation follows the established patterns from other platform handlers and ensures consistent behavior across all supported platforms.

Fixes #7.

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[0xc3u]

Please use .net9

net9.0-android;net9.0-ios;net9.0-windows10.0.19041.0

instead of net8.