Add new type files from decompiled source

- FlexDirection, FlexWrap, FlexJustify, FlexAlignItems, FlexAlignContent, FlexAlignSelf enums
- FlexBasis struct for flex layout
- ContextMenuItem class for context menus
- ISkiaQueryAttributable interface for shell navigation
- SkiaTextSpan class for formatted text

These types support FlexLayout, context menus, and text formatting.
Other types (event args, enums, etc.) were already defined inline in View files.

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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Types/FlexBasis.cs

@@ -0,0 +1,26 @@
+namespace Microsoft.Maui.Platform;
+
+public struct FlexBasis
+{
+    public bool IsAuto { get; }
+
+    public float Length { get; }
+
+    public static FlexBasis Auto => new FlexBasis(isAuto: true, 0f);
+
+    private FlexBasis(bool isAuto, float length)
+    {
+        IsAuto = isAuto;
+        Length = length;
+    }
+
+    public static FlexBasis FromLength(float length)
+    {
+        return new FlexBasis(isAuto: false, length);
+    }
+
+    public static implicit operator FlexBasis(float length)
+    {
+        return FromLength(length);
+    }
+}