Make the Interactive Editor model conform more closely with FileWorkingCopies

[amunger]

after the change to use the simpleNotebookEditorModel #171283, we had to implement some workarounds to avoid the backup service recovering or asking to save Interactive Windows since we've been treating them more as throw-away resources. We should try to integrate better with the working copy service to make use of the functionality provided.

One way to do this is using an untitled resource, so that it wouldn't rely on the dummy file service we created and also wouldn't auto-save. We would also want a setting to close and discard IW resources without confirmation, which may need to be implemented in the backup service.

work-arounds for the current implementation:
#172595
#172427

[amunger]

for the confirmation on editor close, EditorInput can fully override that:

vscode/src/vs/workbench/common/editor/editorInput.ts

Lines 73 to 77 in b2cc8c4

	/**
	* Optional: subclasses can override to implement
	* custom confirmation on close behavior.
	*/
	readonly closeHandler?: IEditorCloseHandler;

[amunger]

completed in #184757