Added std::endian enum

[Chi-Iroh]

Here's C++20 endianness support. Feel free to correct me if my CMake file can be improved, I'm not very familiar with CMake.

[lackhole]

Hi @Chi-Iroh, thank you for the contribution. Can you modify your code to provide a more robustness?

• Static assert is in wrong format. Use following: static_assert(sizeof(char) == 1, "");
• Forward compile options when running endianness compilation/runtime check (i.e., ARM can use both)
• Add a user-settable option for PREVIEW_ENDIAN and don't run configuration if set (i.e., ARM can use both)
• Use program return code rather than relying on read/write (return 0, 1, 2, ...)
• Do not use add_compile_definition. STL-Preview uses configure_file. See cmake_config.h.in and RunConfiguration.cmake
• Write a fallback case for non-CMake configuration. See preview/config.h