Update the exercise structure to use Nimble

Nimble is the [package manager for the Nim programming language](https://github.com/nim-lang/nimble), but also provides a directory structure for the project along with the ability to build the project. As Nimble comes with Nim, we could use this structure to make the test easier to run. We could set up the project to have a "test" task so people would just need to run `nimble test`, like so https://github.com/nim-lang/nimble#tests

I think this would take some work to get to this point and I'm not fully sure what this would look like as far as the test runner, and CI system we have in place right now but I think this would provide a better user experience for someone learning Nim.