Return a chessfriend_moves::EnPassant from a new Position::en_passant()

Replace Position's en_passant_target_square() and en_passant_capture_square()
with a single en_passant() method that returns a new EnPassant struct that has
the target and capture squares for the en passant move.

position/Cargo.toml

@@ -8,3 +8,4 @@ edition = "2021"
 [dependencies]
 chessfriend_core = { path = "../core" }
 chessfriend_bitboard = { path = "../bitboard" }
+chessfriend_moves = { path = "../moves" }