Add grains exercise

[pfertyk]

No description provided.

[pfertyk]

@BNAndras inspired by what you wrote in #28 , I decided to add the "grains" exercise, returning null instead of raising an error. However, it seams that int type in Godot doesn't handle that big numbers ... I guess we will have to skip this one?

[BNAndras]

Yeah, Vim script has the same issue. I think an pragmatic approach would be to reduce the number of squares on the board and add an instructions append highlighting why we reduced the number of squares. So instead of an 8x8 grid, we do a 7x7 grid or smaller.

[BNAndras]

https://forum.exercism.org/t/odd-arithmetic-in-grains/8662 has context and some discussion about this issue for the Vim script track. We ended up marking the status as "wip" as a soft deprecation (only maintainers can see those on the website), and we're still working out a way to solve the problem.

[codingthat]

What about using something like https://github.com/ChronoDK/GodotBigNumberClass ?