Automatically create new constant when dragging and dropping .tscn files into the script editor

[nubels]

It would be a nice UX improvement if you could drag and drop .tscn files into the script editor and have Godot automatically create a constant for you similar to how it works when dragging and dropping nodes while holding control. This way, you would not have to declare the constant.

[Calinou]

Thanks for the proposal! Consolidating in #7673.