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

**Use case**
People are used to Postgres.
{
protected:
    ParserMultiplicativeExpression next_parser;
    const char * getName() const  override { return "DATE operator expression"; }
    bool parseImpl(Pos & pos, ASTPtr & node, Expected & expected) override;
};
class ParserTimestampOperatorExpression : public IParserBase
{
protected:
    ParserDateOperatorExpression next_parser;
    const char * getName() const  override { return "TIMESTAMP operator expression"; }
    bool parseImpl(Pos & pos, ASTPtr & node, Expected & expected) override;
};
class ParserIntervalOperatorExpression : public IParserBase
{
protected:
    ParserTimestampOperatorExpression next_parser;
    const char * getName() const  override { return "INTERVAL operator expression"; }
    bool parseImpl(Pos & pos, ASTPtr & node, Expected & expected) override;
};
class ParserAdditiveExpression : public IParserBase