fix: typeset constructor is now private.

Author: jrfaller

core/src/main/java/com/github/gumtreediff/tree/TypeSet.java

@@ -29,6 +29,8 @@
 public class TypeSet {
     private static final TypeFactoryImplementation implementation = new TypeFactoryImplementation();
 
+    private TypeSet() {}
+
     /**
      * Build a type with the provided name. If the provided name is null or
      * the empty string, the empty type will be returned.