Test C++23 Fixed width floating-point types

[alugowski]

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

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

These likely already work on toolchains that offer both the types and their respective <charconv>.

Possible requirements for testing:

• detect toolchain support for types and charconv support
• if support detected, add optional test_cpp23 that tests these types
• add a runner with a supporting compiler. Probably GCC 13