Support C++23 fixed-width floating point types

[alugowski]

C++23 offers fixed-width floating point types std::float16_t through std::float128_t and std::bfloat16_t.

See https://en.cppreference.com/w/cpp/types/floating-point

This enables support for those types with the assumption that the <charconv> methods support the types. That is true for the only compiler with support for these types, GCC 13. The only exception is std::float128_t, which AFAIK has no string conversion methods in GCC 13's standard library.

Fixes #45