Concern about the definition of `IFormattable`

[RexJaeschke]

Issue #1378 sub-issue 11, was rejected, and during its discussion @KalleOlaviNiemitalo raised a related problem, 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.

[RexJaeschke]

@KalleOlaviNiemitalo feel free to improve the title of this issue and my intro description.