add C++23 examples

Thanks to examples put on developer and C++ reference websites, we have added C++23 examples for compilers that are capable.

A key goal is that CMake correctly and efficiently detect when a compiler is capable of a feature.
Another goal is demonstrating the latest C++ language capabilities of CMake.

We are particularly eager to get GCC and Clang working for C++20 modules in the same way MSVC is, which seems coming in 2023 perhaps.