Class arg_parser

tests/BS_thread_pool_test.cpp:3865–3973

* @brief A class to parse command line arguments. All arguments are simple on/off flags. */

 * @brief A class to parse command line arguments. All arguments are simple on/off flags.
 */
class [[nodiscard]] arg_parser
{
public:
    /**
     * @brief Convert the command line arguments passed to the `main()` function into an `std::vector`.
     *
     * @param argc The number of arguments.
     * @param argv An array containing the arguments.
     */
    arg_parser(int argc, char* argv[]) : args(argv + 1, argv + argc), executable(argv[0]) {};

    /**
     * @brief Check if a specific command line argument has been passed to the program. If no arguments were passed, use the default value instead.
     *
     * @param arg The argument to check for.
     * @return `true` if the argument exists, `false` otherwise.
     */
    [[nodiscard]] bool operator[](const std::string_view arg)
    {
        if (size() > 0)
            return (args.count(arg) == 1);
        return allowed[arg].def;
    }

    /**
     * @brief Add an argument to the list of allowed arguments.
     *
     * @param arg The argument.
     * @param desc The description of the argument.
     * @param def The default value of the argument.
     */
    void add_argument(const std::string_view arg, const std::string_view desc, const bool def)
    {
        allowed[arg] = {desc, def};
    }

    /**
     * @brief Get the name of the executable.
     *
     * @return The name of the executable.
     */
    std::string_view get_executable()
    {
        return executable;
    }

    void show_help() const
    {
        int width = 1;
        for (const auto& [arg, opt] : allowed)
            width = std::max(width, static_cast<int>(arg.size()));
        logln("\nAvailable options (all are on/off and default to off):");
        for (const auto& [arg, opt] : allowed)
            logln("  ", std::left, std::setw(width), arg, "  ", opt.desc);
        log("If no options are entered, the default is:\n  ");
        for (const auto& [arg, opt] : allowed)
        {
            if (opt.def)