C23 support

[wismill]

Requiring C23 now would be a bold move, but maybe we can optionally use some subset of its features if the compiler supports them. constexpr and [[…]] attributes seem nice enhancements.

• https://en.cppreference.com/w/c/compiler_support/23
• https://learn.microsoft.com/en-us/cpp/overview/visual-cpp-language-conformance?view=msvc-170#c-standard-library-features-1