Add support for discriminated union shapes

[eiriktsarpalis]

There should be a type shape kind describing discriminated union like types. This would include polymorphic type hierarchies as well as F# discriminated unions and future C# discriminated unions.

Here's a simplistic sketch of an IUnionShape abstraction:

public interface IUnionTypeShape<TUnion> : ITypeShape<TUnion>
{
    Func<TUnion, int> GetGetUnionCaseIndex();
    IEnumerable<KeyValuePair<string, ITypeShape<TUnion>>> GetUnionCases();
    object? ITypeShape.Accept(ITypeShapeVisitor visitor, object? state) => visitor.VisitUnion(this, state);
}