Fix exception on launch.json with empty configurations

This reverts commit e55b563d, reversing
changes made to 66650300.

This reverts commit ddae0483, reversing
changes made to e55b563d.

Restore untitled files from within UntitledEditorModel

Convert from 1- to 0-based indexing for TypeScript completion spans

Issue #15685

**Bug**

Create an empty project with a `launch.json` like this:

```
{
    "version": "0.2.0",
    "configurations": [
    ]
}
```

Try to start debugging.

See an exception message

**Fix**
Instead of returning undefined for the promise result, return a promise of null like we do in other cases.

closes #15685

Move backup unload logic to an event

Disable backups when extensions are being debugged

Implements current line highlighting with modes 'line', 'gutter' and 'all'

Enhanced diff view