[Concept] Regular Expressions - add exercise templates

[zhravan]

Add exercise templates and tests for Regular Expressions.

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

[aryasoni98]

@zhravan I’m happy to work on this! Could you please assign the issue to me?