Implement checkbox on the vm side (1/2)

Author: yuri-kiss

src/blocks/scratch3_operators.js

@@ -33,10 +33,15 @@ class Scratch3OperatorsBlocks {
             operator_contains: this.contains,
             operator_mod: this.mod,
             operator_round: this.round,
-            operator_mathop: this.mathop
+            operator_mathop: this.mathop,
+            checkbox: this.checkbox
         };
     }
 
+    checkbox () {
+      return true;
+    }
+
     add (args) {
         return Cast.toNumber(args.NUM1) + Cast.toNumber(args.NUM2);
     }