implemented knight legal moves

cpp/src/pieces/knight.cpp

@@ -0,0 +1,28 @@
+#include "../board.hpp"
+#include "../coords.hpp"
+#include "../move.hpp"
+#include "piece.hpp"
+
+#include <vector>
+
+std::vector<Move> knight_moves(const Board& b, const Coords xy) {
+    std::vector<Move> ret;
+    std::vector<std::pair<int, int>> moves = {
+        {+2, +1},
+        {+1, +2}, // north east
+        {+2, -1},
+        {+1, -2}, // south east
+        {-2, -1},
+        {-1, -2}, // south west
+        {-2, +1},
+        {-1, +2} // north west
+    };
+
+    for (const auto& [dx, dy] : moves) {
+        std::optional<Move> move =
+            move_for_position(b, xy, Coords{xy.x + dx, xy.y + dy});
+        if (move.has_value())
+            ret.push_back(move.value());
+    }
+    return ret;
+}