docs: add long string guidance to Dart style guide

[bryanoltman]

Description

Adds a style guide entry about string literals that extend past our line length limit.

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
• [x] 📝 Documentation
• [ ] 🗑️ Chore