support to pass a 'More Information' url from the debug adapter to the frontend

[weinand]

For some errors returned from the debug adapter to the frontend it would be helpful if they could include a link to additional information.

CC @isidorn @chrisdias

[weinand]

The plan is to add a 'url' and a 'urlLabel' attribute to the debug protocol's Message object. The UI would turn this into an action in the error notification.