[Concept] Closing Channels - add exercise templates

[zhravan]

Add exercise templates and tests for Closing Channels.

Acceptance criteria:

• Add templates under internal/exercises/templates/<slug>
• Add tests under internal/exercises/templates/<slug>
• Add to internal/exercises/catalog.yaml
• Provide helpful hints in catalog.yaml

[vtushar06]

Hi @zhravan ,
I am interested in working on this issue as part of Hacktoberfest! Could you please assign it to me?
Thank you!

[kawpii]

@vtushar06 Would still like to work on this issue? If so, I could assign you