Saving games tutorial should show use of var2str and str2var functions

**Your Godot version:**
3.2.2.stable

**Issue description:**
The Saving games tutorial shows how to serialize game state information into json format.

Unfortunately I was having trouble serializing Transform objects into json and ended up writing lots of code to flatten and unflatten this class into lists of 12 numbers.

And then I submitted an issue about it: https://github.com/godotengine/godot-proposals/issues/1351

It turns out that the `var2str()` and `str2var()` functions already handle the case of serializing Variants that are not basic json types.

It would have been useful to have learned about them from reading the tutorial, because they are so relevant.

**URL to the documentation page:**
https://docs.godotengine.org/en/stable/tutorials/io/saving_games.html