feat: improve code logic

[Pantani]

• Scaffold the loan chain with the new coin types;
• Update starport commands into the readme;
• Some code improvements;

[fadeev]

Thanks for suggesting improvements, @Pantani 🙏

@toschdev since this is using unreleased functionality of Starport (scaffolding Coin field). I think we can take these suggestions into account when preparing the future loan tutorial for Starport v0.19.