Allow non-draggable elements on the dropzone

[Fedeorlandau]

If you add any other div that is not present on the items map it will also prevent the dragging, check this example

solves https://github.com/isaacHagoel/svelte-dnd-action/issues/334 and https://github.com/isaacHagoel/svelte-dnd-action/issues/327

I was wondering if we can include a playground folder with svelte to test the changes live.