packery draggabilly: prevent from dropping over a .stamp.non-drop item.

I have a case where I'd like to force an element to always be the first* item where it can not move or be dropped over.

I have tried with stamp but can still drag'n'drop over the stamped item. ideally, it may be nice if 'stamp' could have a `non-drop` option which prevents any item being dragged fro mdropping over it (on drop, it would revert back to the position from where it was picked up). Whilst draggon over an "non-movable stamped item" the item being dragged would have a class classed that may be styled to visually indicate to the user that this item is within a non-drop zone.

thanks in advance,

No file or test is named. Start at Packery's stamp handling and Draggabilly's drag-and-drop entry points, then trace how a stamped item affects placement. Done should cover the requested non-drop behavior, reverting an invalid drop, visual feedback during dragging, and focused tests.

Written by the indexing model from the issue text.