Make a proof of concept teaching with more interactivity and code as used in Godot

_This is a long-term idea, mostly something I intend to try when we remake this course for Godot 4.0 and GDScript 2.0. This won't be worked on soon as:_

_- We're currently limited both in budget and time._
_- We're waiting for Godot 4 to be stable before remaking this app for GDScript 2.0._
_- This will require quite a lot of work to do well._

_But this is something we should work on sometime in 2022, as Godot 4 gets ready._

-----------

In issue #64, goldotasksquestions left us feedback on the course and suggested making text leaner and having more interactivity. Also, a suggestion was to use code that users could reuse as-is in Godot over custom classes, which we currently use to teach specific points (like the LOGO-inspired drawing turtle).

I still think it'll be difficult to make it work for the target audience, i.e. people with 0 prior programming experience, but I'm willing to give it a serious try, make a couple of lessons like this, and run test sessions with beginners.

There are three complementary goals I'd like to pursue with this task with the general goal to improve people's learning experience:

1. Offer a more engaging start experience. The current course starts with a lot of text and lessons get shorter as you go. We could spread the teaching points of the first few lessons in the course.
2. Generally, try to put a little more game design and personality in the app.
- I think one approach that could be appealing would be to put some characters and narrative in the course. Not like lots of dialogues, but a little something that makes the course a bit warmer and makes you want to keep going, be it just to know how things unfold for the characters.
3. Break down learning into smaller chunks. We started doing this with the course's last lessons, but there's still room for improvement.

I think it would have to be a different approach to learning, a more intuitive one, by having people use code they don't understand at first, and have them make sense of it through practice.

A suggested source of inspiration for this is language learning apps, Duolingo in particular. Another is Red Blob Games, a website with in-depth guides about algorithms filled with interactive examples. Other suggestions are welcome.