Support multiple columns in PARTITION BY clause of window function

  @Test public void testSelect() throws SQLException {
    checkSql("model", "select NAME,DEPTNO,count(*) over (partition by NAME,DEPTNO) from EMPS");
  }

windowSpec:
      [ windowName ]
      '('
      [ ORDER BY orderItem [, orderItem ]* ]
      [ PARTITION BY expression [, expression ]* ]
      [
          RANGE numericOrIntervalExpression { PRECEDING | FOLLOWING }
      |   ROWS numericExpression { PRECEDING | FOLLOWING }
      ]
      ')'