Add change function

src/engine/state.js

@@ -15,6 +15,12 @@ class State {
         return this.states.set(key, value);
     }
 
+    change(key, amount = 1) {
+        let val = this.get(key, 0);
+        val += amount;
+        this.set(key, val);
+    }
+
     serialize() {
         const entries = this.states.entries();
         const entrymap = [];