Fix List<T> to ListView<T> casting bug on .NET Framework

[prozolic]

Summary

This PR fixes a bug that occurs only in .NET Framework.

• Sample Code

ObservableList<int> list = new ObservableList<int> { 1, 2, 3, 4, 5 };
var view = list.CreateView(x => x);
list.AddRange(new int[] { 1, 2, 3, 4, 5, 6, 7, 8 });

ObservableList<int> list = new ObservableList<int> { 1, 2, 3, 4, 5 };
list.RemoveRange(1, 3);
list.ForEach(x => Console.Write(x + ","));

The root cause is that List<T> in .NET Framework has a _syncRoot field, but ListView<T> does not include it. As a result, ListView<T>._size is not set correctly. I adjusted the code to use a different ListView<T>(LegacyListView<T>) that accounts for the number of private fields in List<T>, including _syncRoot.

A similar approach is used in ZLinq: https://github.com/Cysharp/ZLinq/blob/main/src/ZLinq/Internal/Polyfill/CollectionsMarshal.cs#L33-L44

Related issue

#109