Write a function that returns all distinct solutions to the 8-queens puzzle.
Each solution contains a distinct board configuration of the 8-queens' placement, where 'Q' and '.' both indicate a queen and an empty space respectively.
Example solution for 4-queen puzzle Output:
// Solution 1
.Q..
...Q
Q...
..Q.
// Solution 2 ..Q. Q... ...Q .Q..
執行方式： dart run main.dart