Supports formatting std::expected<void, E>

[Hackerl]

Consider adding a formatter for std::expected<void, E>：

template <typename E, typename Char>
struct formatter<std::expected<void, E>, Char,
                 std::enable_if_t<is_formattable<E, Char>::value>> {
  FMT_CONSTEXPR auto parse(parse_context<Char>& ctx) -> const Char* {
    return ctx.begin();
  }

  template <typename FormatContext>
  auto format(const std::expected<void, E>& value, FormatContext& ctx) const
      -> decltype(ctx.out()) {
    auto out = ctx.out();

    if (value.has_value()) {
      out = detail::write<Char>(out, "expected(");
    } else {
      out = detail::write<Char>(out, "unexpected(");
      out = detail::write_escaped_alternative<Char>(out, value.error());
    }
    *out++ = ')';
    return out;
  }
};

Output expected() when there is no error