Added Enemy and BrownGuard classes. Added health and damage.

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

lib/classes/enemy.dart

@@ -0,0 +1,23 @@
+import 'package:wolf_dart/classes/entity.dart';
+import 'package:wolf_dart/classes/linear_coordinates.dart';
+
+abstract class Enemy extends Entity {
+  Enemy({
+    required super.x,
+    required super.y,
+    required super.spriteIndex,
+    super.angle,
+    super.state,
+    super.mapId,
+    super.lastActionTime,
+  });
+
+  // Every enemy must implement its own brain!
+  void update({
+    required int elapsedMs,
+    required LinearCoordinates player,
+    required bool Function(int x, int y) isWalkable,
+    required bool Function(Entity entity) hasLineOfSight,
+    required void Function(int damage) onDamagePlayer,
+  });
+}