add guarding mechanic and implement guard sprite handling

src/de/miaurizius/jgame2d/core/enums/Direction.java

@@ -5,6 +5,14 @@ public enum Direction {
     UP,
     DOWN,
     LEFT,
-    RIGHT
+    RIGHT;
 
-}
+    public Direction getOpposite() {
+        return switch (this) {
+            case UP -> DOWN;
+            case DOWN -> UP;
+            case LEFT -> RIGHT;
+            case RIGHT -> LEFT;
+        };
+    }
+}