use new readonly modifier

- VSCode Version: 1.6.0-Insider
- OS Version: Windows

Added the following code to the TS extension when a diagnostic result comes back from the tsserver

``` ts
        if (window.activeTextEditor) {
            window.activeTextEditor.options.lineNumbers = 'relative';
        }
```

Debugged it and the code gets executed but the line numbering doesn't change for the active editor.
