Add collatz-conjecture exercise

[ageron]

Note: I've excluded the test cases for zero or negative inputs.

[Anton-4]

Note: I've excluded the test cases for zero or negative inputs.

How are these handled for other languages?

[ageron]

Some include them, some don't. I initially didn't know what to do with them, so I left them out, but in recent exercises I've adapted the test cases to make them more idiomatic for Roc, i.e., I've use result == Ok 123 and result == Err "some error message.".
Would it be even more idiomatic to use tags instead of strings? E.g., result == Err InvalidNegativeInput or something?

[ageron]

I'll update this exercise to add the error handling test cases.

[ageron]

@Anton-4 , I updated the code to add error handling (see #33), thanks for your feedback.