Add user-defined format string to compose filename

Something on the line of:

/// Format the filename according to base path, prefix, start_number and suffix
std::filesystem::path filename(std::size_t writer_idx = 0)
{
    using namespace fmt::literals;

    return fmt::format("{filename_prefix}_{start_number:05d}{filename_suffix}", "filename_prefix"_a = m_filename_prefix,
                        "file_number"_a = m_start_number + writer_idx,
                        "filename_suffix"_a = m_filename_suffix);
}