Core: Add Item.excludable helper function

Some worlds might want to check for "Item is junk", i.e. an excludable item.

Because this is both `filler` and `trap`, and because `filler` is `0`, there are many "wrong ways" to do this. So I think we should provide a helper function for it.

BaseClasses.py

@@ -1264,6 +1264,10 @@ class Item:
     def trap(self) -> bool:
         return ItemClassification.trap in self.classification
 
+    @property
+    def excludable(self) -> bool:
+        return not (self.advancement or self.useful)
+
     @property
     def flags(self) -> int:
         return self.classification.as_flag()