feat: add app.save_world() function and update documentation

doc/en/scripting/builtins/libapp.md

@@ -83,6 +83,12 @@ app.reopen_world()
 
 Reopens the world.
 
+```lua
+app.save_world()
+```
+
+Saves the world.
+
 ```lua
 app.close_world(
     -- save the world before closing