[explorer, moves, position] Implement a moves command in explorer

The moves command writes all possible moves to the terminal.

Move the previous implementation of the moves command, which marked squares that
a piece could move to, to a 'movement' command.

position/src/lib.rs

@@ -11,4 +11,5 @@ mod macros;
 mod testing;
 
 pub use chessfriend_board::{fen, PlacePieceError, PlacePieceStrategy};
+pub use chessfriend_moves::GeneratedMove;
 pub use position::{Position, ValidateMove};