Add atStackTop helper to Thread

src/engine/thread.js

@@ -123,6 +123,14 @@ Thread.prototype.pushReportedValue = function (value) {
     }
 };
 
+/**
+ * Whether the current execution of a thread is at the top of the stack.
+ * @return {Boolean} True if execution is at top of the stack.
+ */
+Thread.prototype.atStackTop = function () {
+    return this.peekStack() === this.topBlock;
+};
+
 /**
  * Set thread status.
  * @param {number} status Enum representing thread status.