Add restart API.

By: md_5 <git@md-5.net>

paper-api/src/main/java/org/bukkit/Server.java

@@ -1878,6 +1878,13 @@ public interface Server extends PluginMessageRecipient {
         public void broadcast(@NotNull net.md_5.bungee.api.chat.BaseComponent... components) {
             throw new UnsupportedOperationException("Not supported yet.");
         }
+
+        /**
+         * Restart the server. If the server administrator has not configured restarting, the server will stop.
+         */
+        public void restart() {
+            throw new UnsupportedOperationException("Not supported yet.");
+        }
     }
 
     @NotNull