move misc to js/ui

1-js/99-js-misc/01-proxy/03-observable/task.md

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+
+# Observable
+
+Create a function `makeObservable(target)` that "makes the object observable" by returning a proxy.
+
+Here's how it should work:
+
+```js run
+function makeObservable(target) {
+  /* your code */
+}
+
+let user = {};
+user = makeObservable(user);
+
+user.observe((key, value) => {
+  alert(`SET ${key}=${value}`);
+});
+
+user.name = "John"; // alerts: SET name=John
+```
+
+In other words, an object returned by `makeObservable` has the method `observe(handler)`.
+
+Whenever a property changes, `handler(key, value)` is called with the name and value o the property.
+
+
+P.S. In this task, please handle only writing to a property. Other operations can be implemented in a similar way.
+P.P.S. You might want to introduce a global variable or a global structure to store handlers. That's fine here. In real life, such function lives in a module, that has its own global scope.