[Feature] Implement AutomationProperties.HeadingLevel for accessibility

[mtmattei]

f#XXXX### What would you like to be added? 💡

Feature Request

Summary

Implement AutomationProperties.HeadingLevel attached property to support screen reader heading navigation.

Current Behavior

Using AutomationProperties.HeadingLevel produces a build warning and has no effect:

warning Uno0001: Microsoft.UI.Xaml.Automation.AutomationProperties.SetHeadingLevel(
    Microsoft.UI.Xaml.DependencyObject,
    Microsoft.UI.Xaml.Automation.Peers.AutomationHeadingLevel
) is not implemented in Uno

Expected Behavior

HeadingLevel should mark elements as headings (Level1 through Level9) so screen readers can:

• Announce elements as headings
• Allow users to navigate by heading level (a core screen reader feature)

XAML Example

<TextBlock Text="My Tasks"
           Style="{StaticResource HeadlineLarge}"
           AutomationProperties.HeadingLevel="Level1" />

<TextBlock Text="Active Tasks"
           Style="{StaticResource TitleMedium}"
           AutomationProperties.HeadingLevel="Level2" />

Platform Mapping

Platform	Native API
iOS	UIAccessibilityTraits.header
Android	AccessibilityNodeInfo.setHeading(true) (API 28+) or AccessibilityNodeInfoCompat
WASM	role="heading" + aria-level="N"
macOS/Catalyst	NSAccessibility.Role.heading
Skia/Desktop	Platform-specific accessibility APIs

Why This Matters

• WCAG 2.1 Success Criterion 1.3.1 (Info and Relationships) requires heading structure
• WCAG 2.1 Success Criterion 2.4.6 (Headings and Labels) recommends descriptive headings
• Screen reader users rely on heading navigation to understand page structure
• This is a basic accessibility requirement for production apps

WinUI Reference

• AutomationProperties.HeadingLevel Property
• AutomationHeadingLevel Enum

Additional Context

Discovered while building an Uno Platform app following WinUI accessibility best practices. Happy to contribute a PR if guidance is provided on the implementation approach.

Why is this needed? 🤔

Essential for making Uno Platform apps accessible to screen reader users who navigate by heading structure. This is a fundamental accessibility feature that all major platforms support natively.

Renderer 🎨

• Skia
• Native

For which platforms 📱💻🖥️

All platforms 🌍

Anything else we need to know? 💬

This is related to #XXXX (AutomationProperties.HelpText) - both are core AutomationProperties features needed for accessible apps.