Since you're interested in readability, you may also like std::format:

    std::string format_parse_error(std::string base, JSONToken token) {
       std::ostringstream s;
       s << "Unexpected token '" << token.value << "', type '"
         << JSONTokenType_to_string(token.type) << "', index ";
       s << std::endl << base;
      return format_error(s.str(), *token.full_source, token.location);
    }

becomes

    std::string format_parse_error(std::string_view base, JSONToken token) {
      return format_error(
       std::format(
           "Unexpected token '{}', type '{}', index '{}'\n{}"
         , token.value
         , JSONTokenType_to_string(token.type)
         , index
         , base)
       , *token.full_source
       , token.location
      );
    }