Fix all derived Piece methods for generate_legal_moves

[ryan597]

Describe the bug

Currently no implementation of the generate_legal_moves

• Pawn::generate_legal_moves() #7
• Rook::generate_legal_moves() #8
• Knight::generate_legal_moves() #9
• Bishop::generate_legal_moves() #10
• Queen::generate_legal_moves() #11
• King::generate_legal_moves() #12

To Reproduce

Steps to reproduce the behavior:

Expected behavior

Each piece (Pawn, Rook, Knight, Bishop, Queen, King) needs to have its own implementation where the function generate_legal_moves() returns a vector of the valid moves.

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