[Core] Use MessageToNdjsonWriter in MessageFormatter

Follow consistent patterns where all messages are written using
dedicated writers

cucumber-core/src/main/java/io/cucumber/core/plugin/MessageFormatter.java

@@ -1,35 +1,41 @@
 package io.cucumber.core.plugin;
 
+import io.cucumber.messages.MessageToNdjsonWriter;
 import io.cucumber.messages.types.Envelope;
 import io.cucumber.plugin.ConcurrentEventListener;
 import io.cucumber.plugin.event.EventPublisher;
 
 import java.io.IOException;
 import java.io.OutputStream;
-import java.io.Writer;
 
 public final class MessageFormatter implements ConcurrentEventListener {
 
-    private final Writer writer;
+    private final MessageToNdjsonWriter writer;
 
     public MessageFormatter(OutputStream outputStream) {
-        this.writer = new UTF8OutputStreamWriter(outputStream);
+        this.writer = new MessageToNdjsonWriter(outputStream, Jackson.OBJECT_MAPPER::writeValue);
     }
 
     @Override
     public void setEventPublisher(EventPublisher publisher) {
-        publisher.registerHandlerFor(Envelope.class, this::writeMessage);
+        publisher.registerHandlerFor(Envelope.class, this::write);
     }
 
-    private void writeMessage(Envelope envelope) {
+    private void write(Envelope event) {
         try {
-            Jackson.OBJECT_MAPPER.writeValue(writer, envelope);
-            writer.write("\n");
-            if (envelope.getTestRunFinished().isPresent()) {
+            writer.write(event);
+        } catch (IOException e) {
+            throw new IllegalStateException(e);
+        }
+
+        // TODO: Plugins should implement the closable interface
+        // and be closed by Cucumber
+        if (event.getTestRunFinished().isPresent()) {
+            try {
                 writer.close();
+            } catch (IOException e) {
+                throw new IllegalStateException(e);
             }
-        } catch (IOException e) {
-            throw new RuntimeException(e);
         }
     }