[states.base] Add contains for states

solve/states/base.py

@@ -94,6 +94,9 @@ def shapes_available(self, /) -> Multiset[Shape2D]:
         """
         raise NotImplementedError
 
+    def __contains__(self, item: Shape2D, /) -> bool:
+        return item in self.shapes_available
+
     def is_shape_required(self, shape: Shape2D, /) -> bool:
         """
         Return ``True`` if the given shape is required for this state to be done.