[New exercise] Add "Hangman" exercise to the C track

Hello everyone! This is my first time opening an issue here, so if there anything wrong i am doing, please let me know.

I'd like to contribute the Hangman exercise to the C track. It already exists in other languages here on exercisim (Python, Java), so I plan to follow the same structure.

- Is anyone already working on this?
- Do you have specific expectations for structuring exercises in the C track?

If not, may I go ahead and open a PR once done?

Thanks!