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

**Use case**
People are used to Postgres.
{
private:
    Operators_t operators;
    ParserPtr elem_parser;
public:
    /** `operators_` - allowed operators and their corresponding functions
      */
    ParserPrefixUnaryOperatorExpression(Operators_t operators_, ParserPtr && elem_parser_)
        : operators(operators_), elem_parser(std::move(elem_parser_))
    {
    }
protected:
    const char * getName() const override { return "expression with prefix unary operator"; }
    bool parseImpl(Pos & pos, ASTPtr & node, Expected & expected) override;
};
class ParserArrayElementExpression : public IParserBase
{
private:
    static const char * operators[];
protected:
    const char * getName() const  override{ return "array element expression"; }
    bool parseImpl(Pos & pos, ASTPtr & node, Expected & expected) override;
};
class ParserTupleElementExpression : public IParserBase