[windows] improve memory usage of CollectionView

Context: #16436
Context: https://github.com/symbiogenesis/Maui.DataGrid/tree/net8-memory-leak

A customer sample has a `CollectionView` with 150,000 data-bound rows.

Debugging what happens at runtime, I found we effectively were doing:

    _itemTemplateContexts = new List<ItemTemplateContext>(capacity: 150_000);
    for (int n = 0; n < 150_000; n++)
    {
        _itemTemplateContexts.Add(null);
    }

Then the items were created as you scroll each row into view:

    if (_itemTemplateContexts[index] == null)
    {
        _itemTemplateContexts[index] = context = new ItemTemplateContext(_itemTemplate, _itemsSource[index],
            _container, _itemHeight, _itemWidth, _itemSpacing, _mauiContext);
    }
    return _itemTemplateContexts[index];

This code accesses the indexer multiple times, into a `List<T>` with
150,000 `null` items.

To improve this:

* use a `Dictionary<int, T>` instead, just let it size dynamically.

* use `TryGetValue(..., out var context)`, so each call accesses the
  indexer one less time than before.

* use either the bound collection's size or 64 (whichever is smaller) as
  a rough estimate of how many might fit on screen at a time.

Taking a memory snapshot of the app after startup:

    Before:
    Heap Size: 82,899.54 KB
    After:
    Heap Size: 81,768.76 KB

Which is saving about 1MB of memory on launch.

In this case, it feels better to just let the `Dictionary` size itself
with an estimate of what `capacity` will be.

src/Controls/src/Core/Platform/Windows/CollectionView/ItemTemplateContextList.cs

@@ -1,4 +1,5 @@
 #nullable disable
+using System;
 using System.Collections;
 using System.Collections.Generic;
 
@@ -14,21 +15,21 @@ internal class ItemTemplateContextList : IReadOnlyList<ItemTemplateContext>
 		readonly double _itemWidth;
 		readonly Thickness _itemSpacing;
 
-		readonly List<ItemTemplateContext> _itemTemplateContexts;
+		readonly Dictionary<int, ItemTemplateContext> _itemTemplateContexts;
 
 		public int Count => _itemsSource.Count;
 
 		public ItemTemplateContext this[int index]
 		{
 			get
 			{
-				if (_itemTemplateContexts[index] == null)
+				if (!_itemTemplateContexts.TryGetValue(index, out var context))
 				{
-					_itemTemplateContexts[index] = new ItemTemplateContext(_itemTemplate, _itemsSource[index],
+					_itemTemplateContexts[index] = context = new ItemTemplateContext(_itemTemplate, _itemsSource[index],
 						_container, _itemHeight, _itemWidth, _itemSpacing, _mauiContext);
 				}
 
-				return _itemTemplateContexts[index];
+				return context;
 			}
 		}
 
@@ -48,12 +49,7 @@ internal class ItemTemplateContextList : IReadOnlyList<ItemTemplateContext>
 			if (itemSpacing.HasValue)
 				_itemSpacing = itemSpacing.Value;
 
-			_itemTemplateContexts = new List<ItemTemplateContext>(_itemsSource.Count);
-
-			for (int n = 0; n < _itemsSource.Count; n++)
-			{
-				_itemTemplateContexts.Add(null);
-			}
+			_itemTemplateContexts = new(capacity: Math.Max(64, _itemsSource.Count));
 		}
 
 		public IEnumerator<ItemTemplateContext> GetEnumerator()