Add shareware support and spawn correctly for difficulty levels

Signed-off-by: Hans Kokx <hans.d.kokx@gmail.com>

lib/classes/cardinal_direction.dart

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+import 'dart:math' as math;
+
+enum CardinalDirection {
+  east(0.0),
+  south(math.pi / 2),
+  west(math.pi),
+  north(3 * math.pi / 2)
+  ;
+
+  final double radians;
+  const CardinalDirection(this.radians);
+
+  /// Helper to decode Wolf3D enemy directional blocks
+  static CardinalDirection fromEnemyIndex(int index) {
+    switch (index % 4) {
+      case 0:
+        return CardinalDirection.east;
+      case 1:
+        return CardinalDirection.north;
+      case 2:
+        return CardinalDirection.west;
+      case 3:
+        return CardinalDirection.south;
+      default:
+        return CardinalDirection.east;
+    }
+  }
+}

lib/classes/sprite.dart

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+typedef Sprite = List<List<int>>;