refactor classes, add private, minor fixes

1-js/11-async/02-promise-basics/03-animate-circle-promise/task.md

@@ -0,0 +1,15 @@
+
+# Animated circle with promise
+
+Rewrite the `showCircle` function in the solution of the task <info:task/animate-circle-callback> so that it returns a promise instead of accepting a callback.
+
+The new usage:
+
+```js
+showCircle(150, 150, 100).then(div => {
+  div.classList.add('message-ball');
+  div.append("Hello, world!");
+});
+```
+
+Take the solution of the task <info:task/animate-circle-callback> as the base.