Test virtual document provider API

[jrieken]

• any os @bpasero

There is new API to add virtual, readonly document. It works by registering for a url-schema with a provider that can resolve an uri to string content. Make sure that

• the API is easy to understand, yet powerful
• you can give out uris using your scheme and the editor will come back to you
• you can tell the workbench that your virtual document has changed