Discussion: concepts to implement

I was thinking about some other concepts that we don't have at the moment.
I would love to have some opinions. We can leave this issue open to discuss that topic.

## String => #536

That would be a simple one to add, we introduce the concept extremely quickly in `basics-2` but mention almost nothing about the library. It's a very rich library, it would be nice to touch upon functions that are a little bit different in other languages, maybe like `cons`, `uncons`, `words`, `toX`, `fromX`...

## Regex => ⛔

I know it's not used very often in Elm, but there are still cases where it's useful, and we provide the library, so why not. We can fork an existing concept for this, [like this one](https://github.com/exercism/go/blob/main/exercises/concept/parsing-log-files/.docs/instructions.md).

## Json Decoders => #538

This is usually a pain point for beginners, because the approach is very unique. Well worth a concept. There is [one exercise](https://github.com/exercism/problem-specifications/blob/main/exercises/rest-api/description.md) using json input.

## Random => #537

There are a few exercises that need it, and it's not an obvious thing in Elm, so it might be good to introduce.
The problem is that it's not possible to use without `Platform` functions, and we cannot test that without `elm-program-test` so we would need to add the dependency. However with this, we can introduce effectful concepts, like `Task` or the Elm architecture.