Add Visual Studio 2013 solution and project files under util/msvc2013/ for both wren CLI and wren_lib static library, including proper source file filters and include paths. Update .gitignore to allow the new project directories while still ignoring Debug/Release outputs. Fix io.c to include <fcntl.h> on _WIN32 for O_RDONLY definition, replace S_IRUSR with 0 for cross-platform uv_fs_open compatibility, and correct uv_buf_t access to use request->fs.info.bufs for libuv API changes. Remove references to wren_io.c and wren_meta.c from the library project, replacing them with wren_opt_meta.c and wren_opt_random.c from the new optional source directory.
