Square constructor
It creates a 6x6 matrix of zeros
Implementation
Square() {
grid = <List<int>>[
<int>[0, 0, 0, 0, 0, 0],
<int>[0, 0, 0, 0, 0, 0],
<int>[0, 0, 0, 0, 0, 0],
<int>[0, 0, 0, 0, 0, 0],
<int>[0, 0, 0, 0, 0, 0],
<int>[0, 0, 0, 0, 0, 0],
];
}