fix: thickness converter possible null reference return

[United600]

Minor refactor and improvement to the ThicknessFiltersConverter class and its related enum.

• Adds new ThicknessFilterKinds combinations
• Renames the Convert method to Extract
• Sets the Extract method to static for direct XAML usage
• Changes the Extract behavior, the None value now sets all fields to 0
• Improves the Convert method by returning DependencyProperty.UnsetValue instead of null for unsupported values
• Sets the Filters property default value back to None
• Removes the partial modifier, as it will no longer be needed for UWP modern NET migration

[Copilot]

Pull Request Overview

This PR refactors the ThicknessFiltersConverter class to improve its API design and behavior. The changes focus on adding new thickness filter combinations, improving method naming, and making the converter more suitable for XAML usage.

• Adds new ThicknessFilterKinds enum values (Horizontal, Vertical, All) for better filtering options
• Renames Convert method to Extract and makes it static for direct XAML usage
• Changes behavior to return DependencyProperty.UnsetValue instead of null for invalid inputs

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[huynhsontung]

I'm not aware of the DependencyProperty.UnsetValue. Is there a functional difference between the two?

[United600]

Same here, I only learned about it from this toolkit converter. Not sure about the difference, we could just add #nullable enable to the file and call it a day. I'm not sure which one is the "correct".