Concern about the definition of `IFormattable`

Issue https://github.com/dotnet/csharpstandard/issues/1378 [sub-issue 11](https://github.com/dotnet/csharpstandard/issues/1378#issuecomment-3180208256), was rejected, and during its discussion @KalleOlaviNiemitalo raised a [related problem](https://github.com/dotnet/csharpstandard/issues/1378#issuecomment-3180230795), which was spun off to this new Issue. He wrote:

> I have doubts about the `IFormattable` definition, which currently doesn't include the `string ToString(string? format, IFormatProvider? formatProvider)` method. That makes it impossible for a conforming program to implement the method as an explicit interface implementation. And I'm not sure a conforming implementation can add the method, because `IFormattable` is an interface.