refactor: split "Strings" lesson

[PoetaKodu]

Description

Split the "Strings" lesson into smaller ones.

Type of Change

• [ ] ✨ New feature (non-breaking change which adds functionality)
• [ ] 🛠️ Bug fix (non-breaking change which fixes an issue)
• [ ] ❌ Breaking change (fix or feature that would cause existing functionality to change)
• [ ] 🧹 Code refactor
• [ ] ✅ Build configuration change
• [ ] 📝 Documentation
• [ ] 🇺🇸 Translation
• [ ] 🗑️ Chore