fix: Disallow mutatation of SpriteAnimationGroupComponent.animations (

#3183)

Many users try to modify the `SpriteAnimationGroupComponent.animations`
map, but that doesn't work since it has to go through the setter, so
this PR makes the returned map from the getter unmodifiable.

packages/flame/lib/src/components/sprite_animation_group_component.dart

@@ -151,7 +151,11 @@ class SpriteAnimationGroupComponent<T> extends PositionComponent
   }
 
   /// Returns the map of animation state and their corresponding animations.
-  Map<T, SpriteAnimation>? get animations => _animations;
+  ///
+  /// If you want to change the contents of the map use the animations setter
+  /// and pass in a new map of animations
+  Map<T, SpriteAnimation>? get animations =>
+      _animations != null ? Map.unmodifiable(_animations!) : null;
 
   /// Sets the given [value] as new animation state map.
   set animations(Map<T, SpriteAnimation>? value) {