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

**Use case**
People are used to Postgres.
{
        ParserToken sep_parser{separator};
        return parseUtil(pos, expected, parse_element, sep_parser, allow_empty_);
    }
    template <typename F>
    static bool parseUtil(Pos & pos, Expected & expected, const F & parse_element, bool allow_empty_ = true)
    {
        return parseUtil(pos, expected, parse_element, TokenType::Comma, allow_empty_);
    }
protected:
    const char * getName() const override { return "list of elements"; }
    bool parseImpl(Pos & pos, ASTPtr & node, Expected & expected) override;
private:
    ParserPtr elem_parser;
    ParserPtr separator_parser;
    bool allow_empty;
    char result_separator;
};
/** An expression with an infix binary left-associative operator.
  * For example, a + b - c + d.
  */
class ParserLeftAssociativeBinaryOperatorList : public IParserBase