make chest's gold depend on level

src/artifacts.rs

@@ -22,9 +22,11 @@ pub struct Chest {
 
 impl Chest {
     pub fn new(position: Position) -> Self {
+        let level = position.get_level();
         Self {
             position,
-            gold: 10,
+            // TODO maybe randomize this?
+            gold: (level + 1) * 10,
         }
     }
 }