feat: allow dropping a folder into the dropzone

[gvdp]

Recreated https://github.com/adopted-ember-addons/ember-file-upload/pull/834 after updating master with all the latest changes.

As for the question about how in-demand this is: we've needed it for one of our apps and it's been in production for over a year now. I made it so there is an opt-in boolean which needs to be enabled, so by default nothing should change for exisiting users.

Seems also like this is one of the oldest issues on this repo where some people have asked for it: https://github.com/adopted-ember-addons/ember-file-upload/issues/2