`INTERVAL '1 hour'` should be equivalent to `INTERVAL 1 HOUR`

**Use case**
People are used to Postgres.
: elem_parser(std::move(elem_parser_))
        , separator_parser(std::move(separator_parser_))
        , allow_empty(allow_empty_)
        , result_separator(result_separator_)
    {
    }
    template <typename F>
    static bool parseUtil(Pos & pos, Expected & expected, const F & parse_element, IParser & separator_parser_, bool allow_empty_ = true)
    {
        Pos begin = pos;
        if (!parse_element())
        {
            pos = begin;
            return allow_empty_;
        }
        while (true)
        {
            begin = pos;
            if (!separator_parser_.ignore(pos, expected) || !parse_element())
            {
                pos = begin;
                return true;
            }
        }
        return false;
    }
    template <typename F>
    static bool parseUtil(Pos & pos, Expected & expected, const F & parse_element, TokenType separator, bool allow_empty_ = true)