Other: Replaced the `getResizeObserver()` helper with the `ResizeObserver` class for performance reasons. Closes ckeditor/ckeditor5#6145.
MINOR BREAKING CHANGE: The `getResizeObserver()` helper is no longer available. We recommend using the [`ResizeObserver`](http://ckeditor.com/docs/ckeditor5/latest/api/module_utils_dom_resizeobserver-ResizeObserver.html) class instead.