initLockedStderr

fn initLockedStderr(t: *Threaded, terminal_mode: ?Io.Terminal.Mode) Io.Cancelable!Io.LockedStderr {
    if (!t.stderr_writer_initialized) {
        const io_t = io(t);
        if (is_windows) t.stderr_writer.file = .stderr();
        t.stderr_writer.io = io_t;
        t.stderr_writer_initialized = true;
        t.scanEnviron();
        const NO_COLOR = t.environ.exist.NO_COLOR;
        const CLICOLOR_FORCE = t.environ.exist.CLICOLOR_FORCE;
        t.stderr_mode = terminal_mode orelse try .detect(io_t, t.stderr_writer.file, NO_COLOR, CLICOLOR_FORCE);
    }
    return .{
        .file_writer = &t.stderr_writer,
        .terminal_mode = terminal_mode orelse t.stderr_mode,
    };
}