Fix #135: Identify package names in extension clause

[nicolasstucki]

No description provided.

[MaximeKjaer]

Two small things stand out to me here:

1. The dots aren't highlighted. I think we should aim to highlight the dots in the qualified name as punctuation, like we do for qualified names in imports.
2. The package names are highlighted as scala.source, whereas they are highlighted as entity.name.import.scala for imports. With my editor theme, package names show up as yellow in imports, and white in extends. Should we aim to unify the two? For instance, the package names in extends could be entity.name.scala.

[nicolasstucki]

The unification idea sounds good but I would do it on it as a separate enhancement. The current fix targets a simple but common issue that I which to fix before GitHub updates the syntax adding this bug.

I will open a separate issue for the unification idea.

[MaximeKjaer]

Perfect, I see you've opened #143. Since we're tracking that issue, we can merge this now.