Add Practice Exercise: `triangle`

[0xNeshi]

Towards https://github.com/exercism/cairo/issues/113

Problem specification: https://github.com/exercism/problem-specifications/tree/main/exercises/triangle

TODO:

• generate the exercise with ./bin/fetch-configlet && ./bin/configlet create --practice-exercise triangle
• check if any of the existing tracks (examples below) implement the exercise and you can reuse the implementation:
  • Go: https://github.com/exercism/go/tree/main/exercises/practice
  • Rust: https://github.com/exercism/rust/tree/main/exercises/practice
  • Python: https://github.com/exercism/python/tree/main/exercises/practice
• use exercise's canonical-data.json to see which test cases to implement: https://github.com/exercism/problem-specifications/blob/main/exercises/triangle/canonical-data.json

[BNAndras]

For this exercise, there are generally two approaches I've seen for implementation. We provide three functions that check if the sides correspond to a particular kind of triangle, returning true or false (see https://github.com/exercism/csharp/blob/main/exercises/practice/triangle/Triangle.cs). The other approach is that we have a single function that checks the sides and returns what kind of triangle it is (see https://github.com/exercism/elixir/blob/main/exercises/practice/triangle/lib/triangle.ex). In the later case, some of the tests should be excluded where a triangle may be more than one type based on the sides.

Any preference?

[0xNeshi]

Let's go with the former version, as it aligns with the "official" problem specification (https://github.com/exercism/problem-specifications/blob/main/exercises/triangle/canonical-data.json).