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

**Use case**
People are used to Postgres.
{
private:
    static const char * operators[];
    ParserLeftAssociativeBinaryOperatorList operator_parser {operators, std::make_unique<ParserIntervalOperatorExpression>()};
protected:
    const char * getName() const  override { return "additive expression"; }
    bool parseImpl(Pos & pos, ASTPtr & node, Expected & expected) override
    {
        return operator_parser.parse(pos, node, expected);
    }
};
class ParserConcatExpression : public IParserBase
{
    ParserVariableArityOperatorList operator_parser {"||", "concat", std::make_unique<ParserAdditiveExpression>()};
protected:
    const char * getName() const override { return "string concatenation expression"; }
    bool parseImpl(Pos & pos, ASTPtr & node, Expected & expected) override
    {
        return operator_parser.parse(pos, node, expected);
    }
};
class ParserBetweenExpression : public IParserBase
{
private:
    ParserConcatExpression elem_parser;
protected:
    const char * getName() const override { return "BETWEEN expression"; }
    bool parseImpl(Pos & pos, ASTPtr & node, Expected & expected) override;
};
class ParserComparisonExpression : public IParserBase