Doesn't work with additional language

[garlen-javier]

I tried this plugin together with gatsby-remark-vscode, other language are working fine but seems it is not working with added language like dart.

Example:

```dart:title=main.dart
void main() =>
    runApp(
      DevicePreview(
        enabled: true,
        builder: (context) => MyApp(),
      ),
    );

Output: