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/FlexAlignItems.cs

@@ -0,0 +1,9 @@
+namespace Microsoft.Maui.Platform;
+
+public enum FlexAlignItems
+{
+    Start,
+    Center,
+    End,
+    Stretch
+}