feat: improve drop check callbacks by adding both nodes

[sashokbg]

Hello,

I am proposing this minor update of both dropTarget and dropSiblings callbacks:

By passing both the grabbed node and the node target, we allow for a more refined conditions on which node can be dropped upon which other.

I kept the first argument the "target" node to insure compatibility for users who already use these callbacks

[maxkfranz]

Looks good