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

**Use case**
People are used to Postgres.
{
private:
    Operators_t operators;
    ParserPtr first_elem_parser;
    ParserPtr remaining_elem_parser;
public:
    /** `operators_` - allowed operators and their corresponding functions
      */
    ParserLeftAssociativeBinaryOperatorList(Operators_t operators_, ParserPtr && first_elem_parser_)
        : operators(operators_), first_elem_parser(std::move(first_elem_parser_))
    {
    }
    ParserLeftAssociativeBinaryOperatorList(Operators_t operators_, ParserPtr && first_elem_parser_,
        ParserPtr && remaining_elem_parser_)
        : operators(operators_), first_elem_parser(std::move(first_elem_parser_)),
          remaining_elem_parser(std::move(remaining_elem_parser_))
    {
    }
protected:
    const char * getName() const override { return "list, delimited by binary operators"; }
    bool parseImpl(Pos & pos, ASTPtr & node, Expected & expected) override;
};
/** Expression with an infix operator of arbitrary arity.
  * For example, a AND b AND c AND d.
  */
class ParserVariableArityOperatorList : public IParserBase