Allow elements within wp.Uploader dropzones to be repositioned on dragenter

[koopersmith]

While working on #21390, I found that when attempting to reposition an element inside of a dropzone, browsers (namely Webkit-based browsers) will quickly fire the dragleave event, immediately followed by a dragover (which we're using to simulate dragenter), as the target of the event is quickly changed, but the ancestor we care about is still the same.

Using an immediately executed setTimeout prevents the drag-over class from being quickly removed and re-added when elements inside the dropzone are repositioned.

[koopersmith]

In [21630]:

Allow elements within a wp.Uploader dropzone to be repositioned when the drag-over class is added. fixes #21705.