Make model editing API public, fixes #364

Still missing:

- Detailed documentation.
- Python bindings.

PiperOrigin-RevId: 641445626
Change-Id: I20e67b707cf1bebae7e0cc94d17f7b76a89171f0

src/xml/xml_native_writer.cc

@@ -22,11 +22,12 @@
 
 #include <mujoco/mjmodel.h>
 #include <mujoco/mjplugin.h>
+#include <mujoco/mjspec.h>
+#include <mujoco/mujoco.h>
 #include "engine/engine_io.h"
 #include "engine/engine_plugin.h"
 #include "engine/engine_util_errmem.h"
 #include "engine/engine_util_misc.h"
-#include "user/user_api.h"
 #include "user/user_model.h"
 #include "user/user_objects.h"
 #include "user/user_util.h"