Missing support for std::format

[fekir]

Description

doctest does not seem to be able to output variables that can be formatted with std::format, but do not have a stream operator

Steps to reproduce

See example at https://godbolt.org/z/5Enb7bs73

doctest prints {?}

Extra information

doctest trunk on compiler explorer

[fekir]

Additional note: it would be nice if the library format is supported to, in that case, it is possible to define a formatter with a friend function format_as.