Add body.find_all() to Python bindings.

This enables to generate a list of all body children of a given type.

Also, add recursive option to mjs_firstChild and mjs_nextChild. Fixes #2112.

PiperOrigin-RevId: 681039348
Change-Id: Iebbb26fd5ec314150206edfdddd88633baa0bec6

src/user/user_objects.cc

@@ -1338,6 +1338,9 @@ mjsElement* mjCBody::NextChild(mjsElement* child, mjtObj type, bool recursive) {
       candidate = GetNext(frames, child, recursive);
       break;
     default:
+      throw mjCError(this,
+                     "Body.NextChild supports the types: body, frame, geom, "
+                     "site, light, camera");
       break;
   }