triangle: update starter implementation

exercises/triangle/src/main/java/Triangle.java

+class Triangle {
+
+    Triangle(double side1, double side2, double side3) throws TriangleException {
+        throw new UnsupportedOperationException("Delete this statement and write your own implementation.");
+    }
+
+    TriangleKind getKind() {
+        throw new UnsupportedOperationException("Delete this statement and write your own implementation.");
+    }
+
+}