Add access to dropEffect for dropTarget

[user1736]

Hi there!

Is your feature request related to a problem? Please describe.
I'd like to have an access to the current drop effect in canDrop method within dropTarget decorator spec, so I could disable/enable drop targets according to current effect.

Describe the solution you'd like
I think it'll be nice to have an access to dropEffect from monitor. I prepared draft solution for version 3.0.2 here and don't mind adding this feature to newer versions of the library if needed.

It might be useful in #974 as well

[user1736]

Up! Any comments on that? At least could you please triage it.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[willurmston]

This feature would be super helpful to me. I need to know if the dropped item was moved or copied from within the DropTarget, but from what I've read, you can only access the dropEffect in the DragSource. I have many DropTargets that all lead to different places, so I prefer to handle the onDrop logic in the DropTarget. Maybe I'm using the library incorrectly, but this seems like a valid use case.