The runtime should expose the following feature flag for runtimes that support static abstract members in interfaces.

namespace System.Runtime.CompilerServices
{
    public static class RuntimeFeature
    {
        public const string VirtualStaticsInInterfaces = nameof(VirtualStaticsInInterfaces);
    }
}

The implementation of static bool IsSupported(string feature) should be updated accordingly.

FYI @davidwrighton @AlekseyTs

Relates to language feature: dotnet/csharplang#4436

Per discussion with the overall feature crew, we'd like this change in .NET 6 preview 5, to line up with runtime and C# compiler work.