Add drag and drop of explorer objects into editor

[ScriptPup]

Is your feature request related to a problem? Please describe.
Users are requesting the ability to drag and drop connection items from the sidebar into their editor.
ScriptPup/sqltools-teradata-driver#10

Describe the solution you'd like
Allow users to drag-and-drop items from the connection bar into the editor.

Describe alternatives you've considered
N/A

Additional context
Essentially, this is the same request as way back in 2017, but now VSCode supports it!

• MSFT support added TreeDragAndDropController.
• MSFT support added DocumentDropEditProvider
• MSFT fixed a few bugs causing dragging and dropping from tree to custom editor to not work

So far I've had trouble finding specific examples of how to bridge the two (of course, Microsoft is notoriously bad about giving a ton of documentation with absolutely nothing practical/concrete to get one started).

But I plan to make a PR for this feature once I figure it out... About to get started on it now.

[ScriptPup]

I think the bugs mentioned in the linked posts above might still be present, as I'm able to set the data transfer but when I retrieve the custom mime-type the value is blank. If I set, then receive within the same call I can see the data was set correctly. So it seems like there's some kind of disconnect with the retrieval.

I'm still playing with it to see if there's some magic combination. If not I'll have to make a minimal example of what I'm trying to accomplish and post a bug to the vscode repo.