first_2d_game using godot-cpp

[j-dax]

Your Godot version:
4.3.stable.mono.official.77dcf97d8

Issue description:
Enhance tutorial getting_started/first_2d_game to include C++ extensions using godot-cpp (as suggested in the tutorial as a future enhancement).

URL to the documentation page (if already existing):
https://docs.godotengine.org/en/stable/getting_started/first_2d_game/01.project_setup.html

I have written a working version of this project and would like to incorporate it to the docs. In the next couple days, I'll go through a second time to better document and correct discrepancies, then work on a pull request.

I wanted to open this issue to discuss what changes should be made in the docs including the granularity of code blocks as well as conventions we should use in C++. As previously discussed by others in #3390, I think this would be a nice resource for those getting started with godot-cpp extensions.

[tetrapod00]

For reference, you might want to look at the previous version of the First 2D Game C++ tutorial. It was added in #5078, and removed in #8473. You can still see it in the 3.5 branch.

Since this would be a pretty big change that also adds a bit of support and maintenance expectations, you might also want to discuss this in the docs channel of the contributors chat. There is a chance that keeping a C++ version of the First 2D Game tutorial is no longer in scope for the docs.