filedragenter/filedragleave are both emitted whilst in the drop zone#6

First mentioned here: #5 From what I can understand from the docs, I think filedragenter and filedragleave are emitted too often. I assume that `filedragenter `should only be emitted once, when the user's cursor _enters_ the dropzone while dragging some files. The reverse should be true for filedragleave. In this [REPL ](https://svelte.dev/repl/1d4838df20fe4a3f8e9b1b3ac9ed47f2?version=3.44.3)(fixed), you can observe the console logs by opening your DevTools. If you drag some files to the drop zone (don't release) and start moving your mouse up and down inside the drop zone, events are emitted (both enter and leave events).