add two methods to the undomanager

Expose two methods on the undo manager class, `isClean` and `markClean`. These don't show up in the html docs, but can be seen here: https://github.com/ajaxorg/ace/blob/master/lib/ace/undomanager.js#L152

Cheers!
Looks like a type here since `markClean` is a method?  I think this needs to be one of:

```dart
_proxy['markClean'] = () => markClean();
_proxy['markClean'] = markClean;
```

I'd lean towards the latter probably, as it is one less closure.