Add dev server with livereload

npm run dev starts a dev server on port 8080 serving the test
page and ribbit dist files. Watches src/ and test/integration/
for changes, rebuilds automatically, and notifies connected
browsers to reload via EventSource on port 8081.

The test page includes a livereload script that auto-reloads
when the dev server signals a rebuild.

test/integration/index.html

@@ -42,5 +42,10 @@
         editor.run();
         window.__ribbitEditor = editor;
     </script>
+    <script>
+        // Livereload — connects to dev server's EventSource endpoint.
+        // Silently fails if the dev server isn't running.
+        try { new EventSource('http://localhost:8081').onmessage = () => location.reload(); } catch(e) {}
+    </script>
 </body>
 </html>