Treat Entire Sidebar as Drop Zone

[curran]

Following https://github.com/vizhub-core/vzcode/pull/575/files, I think it would make sense to treat the entire sidebar as the drop zone, rather than have a specific drop zone called out in the UI. I think that would be cleaner in terms of UI, and also that's what other editors like VSCode do.

The drop zone callout in the UI is good in a way, because it makes the feature discoverable, but my preference would be to keep the UI clean, and use the entire sidebar as the drop zone. It should be possible to do this by migrating DragAndDrop from a React component to a custom hook, maybe called useDragAndDrop, which would return the things used here:

      className={`drop-zone ${isDragOver ? 'dragover' : ''}`}
      onDragEnter={handleDragEnter}
      onDragOver={handleDragOver}
      onDragLeave={handleDragLeave}
      onDrop={handleDrop}

then those props could be applied to the sidebar itself. I do like the dragover style - that could apply to the entire sidebar.

[curran]

Done in #587