Add more efficient functions to check for files or directories on Windows (bug #59711). * liboctave/system/lo-sysdep.h, liboctave/system/lo-sysdep.cc (octave::sys::file_exists, octave::sys::dir_exists): Calling `stat` on Windows is inefficient because it isn't a native function. Use native API functions to check if a file or directory with a given name exists.