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

**Use case**
People are used to Postgres.
{
private:
    const char * infix;
    const char * function_name;
    ParserPtr elem_parser;
public:
    ParserVariableArityOperatorList(const char * infix_, const char * function_, ParserPtr && elem_parser_)
        : infix(infix_), function_name(function_), elem_parser(std::move(elem_parser_))
    {
    }
protected:
    const char * getName() const override { return "list, delimited by operator of variable arity"; }
    bool parseImpl(Pos & pos, ASTPtr & node, Expected & expected) override;
};
/** An expression with a prefix unary operator.
  * Example, NOT x.
  */
class ParserPrefixUnaryOperatorExpression : public IParserBase