Allow the placement of the cursor in a DAP completion item

For arrays, I have a completion item that appears as `[index]` and, when selected, completes

```
arr.|
```

to:

```
arr[|
```

Ideally I'd like to be able to tack on the closing brace and place the cursor inside the array:

```
arr[|]
```

Or complete it as `arr[index]` and select word "index". Adding a new selection range property to the `CompletionItem` would let us do this.