Registering xt in the Resource Service Registry

Hi Moritz,

this is a proposal on how the Resource Service Registry can be filled for "xt" - I'm simply reusing the instance from "xpect" here. However, I'm not really sure if that's the correct way to go, so could you please have a look?

Thanks,
Simon

---
*Background:*
The "xt" DSL does not have an editor (anymore), so there is no corresponding entry in the plugin.xml
that would make the Standalone Registry create an entry for "xt" in the Resource Service Registry. As a result
```java
resourceServiceProviderRegistry.getResourceServiceProvider(resource.getURI())
```
always returned ```null```for "*.xt" resources during test execution.