Polyfill for union types

[virzak]

Could we have this code for .NET10 and below?

namespace System.Runtime.CompilerServices
{
    [AttributeUsage(AttributeTargets.Class | AttributeTargets.Struct,
        AllowMultiple = false)]
    public sealed class UnionAttribute : Attribute;

    public interface IUnion
    {
        object? Value { get; }
    }
}

Looks like it will be added to .NET 11 in the later previews:

https://devblogs.microsoft.com/dotnet/csharp-15-union-types/#try-it-yourself