perf: Improve nullability checking in DependencyProperty

Move nullability checking to use `HashTableEx` to avoid
JIT/Generics costs when initializing DependencyProperty nullability
validation. The original implementation is in Uno.Core, but uses
ConcurrentDictionary, which is particularly expensive to initialize
in Jitted and Full AOT environments.

Author: jeromelaban

src/Uno.UI/UI/Xaml/DependencyProperty.Dictionary.TypeNullableDictionary.cs

@@ -0,0 +1,47 @@
+#nullable enable
+
+using System;
+using Uno.Collections;
+using Uno.UI.Helpers;
+using Uno.Extensions;
+
+namespace Windows.UI.Xaml
+{
+	public sealed partial class DependencyProperty
+	{
+		private readonly static TypeNullableDictionary _isTypeNullableDictionary = new TypeNullableDictionary();
+
+		private bool GetIsTypeNullable(Type type)
+		{
+			if(!_isTypeNullableDictionary.TryGetValue(type, out var isNullable))
+			{
+				_isTypeNullableDictionary.Add(type, isNullable = type.IsNullable());
+			}
+
+			return isNullable;
+		}
+
+		private class TypeNullableDictionary
+		{
+			private readonly HashtableEx _entries = new HashtableEx(FastTypeComparer.Default);
+
+			internal bool TryGetValue(Type key, out bool result)
+			{
+				if (_entries.TryGetValue(key, out var value))
+				{
+					result = (bool)value!;
+					return true;
+				}
+
+				result = false;
+				return false;
+			}
+
+			internal void Add(Type key, bool isNullable)
+				=> _entries.Add(key, isNullable);
+
+			internal void Clear()
+				=> _entries.Clear();
+		}
+	}
+}

src/Uno.UI/UI/Xaml/DependencyProperty.cs

@@ -55,7 +55,7 @@ private DependencyProperty(string name, Type propertyType, Type ownerType, Prope
 			_ownerType = ownerType;
 			_isAttached = attached;
 			_isDependencyObjectCollection = typeof(DependencyObjectCollection).IsAssignableFrom(propertyType);
-			_isTypeNullable = propertyType.IsNullableCached();
+			_isTypeNullable = GetIsTypeNullable(propertyType);
 			_uniqueId = Interlocked.Increment(ref _globalId);
 			_hasWeakStorage = (defaultMetadata as FrameworkPropertyMetadata)?.Options.HasWeakStorage() ?? false;