Add World.save() and Server.savePlayers();

By: Morgan Humes <mrh9724@lanaddict.com>

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

@@ -130,4 +130,9 @@ public interface Server {
      * @return PluginCommand if found, otherwise null
      */
     public PluginCommand getPluginCommand(String name);
+
+    /**
+     * Writes loaded players to disk
+     */
+    public void savePlayers();
 }