Connect Four board with gravity drops and board snapshots

Build the board for a gravity-based four-in-a-row game. Players drop single-character colored pieces into columns. A piece falls straight down and stops in the lowest empty cell of its column.

Implement ConnectFourBoard:

• ConnectFourBoard(rows, cols) creates an empty board with rows rows and cols columns. Every cell starts as the character '0'. Row 0 is the top row, and row rows - 1 is the bottom.
• drop(color, col) -> List[List[str]] drops a piece of color into column col, then returns the board after the move as a 2D list of characters.
  • color is any single character other than '0', for example 'R', 'Y' or 'G'.
  • If column col is already full, the board does not change, and you still return its current state.
  • The returned grid must be an independent copy. Changes the caller makes to it must not affect the board, and later drops must not change a grid returned earlier.
  • Raise an error if col is outside 0..cols-1.

Part 2. Add last_move_wins() -> bool. It returns true when the piece placed by the most recent successful drop forms a line of at least 4 pieces of its color: horizontal, vertical or diagonal. It returns false if the last drop hit a full column.

Constraints. 1 <= rows, cols <= 100. There are at most 10^4 calls to drop.

Example 1

b = ConnectFourBoard(3, 3)
b.drop('G', 2)  ->  [['0','0','0'],
                     ['0','0','0'],
                     ['0','0','G']]
b.drop('B', 2)  ->  [['0','0','0'],
                     ['0','0','B'],
                     ['0','0','G']]

Example 2

b = ConnectFourBoard(2, 2)
b.drop('G', 0)
b.drop('B', 0)
b.drop('G', 0)  ->  [['B','0'],
                     ['G','0']]   # column 0 is full, so nothing changes

Reconstructed from the reported question; details may differ from what interviewers ask.