`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<ParserBetweenExpression>()};
protected:
    const char * getName() const  override{ return "comparison expression"; }
    bool parseImpl(Pos & pos, ASTPtr & node, Expected & expected) override
    {
        return operator_parser.parse(pos, node, expected);
    }
};
/** Parser for nullity checking with IS (NOT) NULL.
  */
class ParserNullityChecking : public IParserBase
{
private:
    ParserComparisonExpression elem_parser;
protected:
    const char * getName() const override { return "nullity checking"; }
    bool parseImpl(Pos & pos, ASTPtr & node, Expected & expected) override;
};
class ParserLogicalNotExpression : public IParserBase
{
private:
    static const char * operators[];
    ParserPrefixUnaryOperatorExpression operator_parser {operators, std::make_unique<ParserNullityChecking>()};
protected:
    const char * getName() const  override{ return "logical-NOT expression"; }
    bool parseImpl(Pos & pos, ASTPtr & node, Expected & expected) override