Add missing parser for extern types (#1300)

Author: phischu

effekt/jvm/src/test/scala/effekt/core/ReparseTests.scala

@@ -84,11 +84,11 @@ class ReparseTests extends CoreTests {
     val printed = core.ReparsablePrettyPrinter.format(expectedRenamed).layout
     val reparsed: ModuleDecl = try { parse(printed)(using Location.empty) } catch {
       case e @ TypeError(msg, context) =>
-         println(e.toString)
-         throw e
+        println(e.toString)
+        throw e
       case e =>
         println("Error while re-parsing:")
-        println(printed)
+        println(e)
         throw e
     }
     val reparsedRenamed = renamer(reparsed)

effekt/shared/src/main/scala/effekt/core/Parser.scala

@@ -309,6 +309,9 @@ class CoreParsers(names: Names) extends EffektLexers {
           id, tparams, cparams, vparams, bparams, result, captures,
           ExternBody.StringExternBody(ff, templ)
         )
+    }
+    | `extern` ~> `type` ~> id ~ typeParams.? ^^ {
+      case id ~ tparams => Extern.Data(id, tparams.getOrElse(Nil))
     })
 
   lazy val externBodyTemplate: P[Template[Expr]] =