generalize implementation for Dollar.times

Tasklist:
- $5 + 10CHF = $10 if rate is 2:1 🎯
- $5 * 2 = $10 ✅
- Make "amount" private
- Dollar side-effects?
- Money rounding?

src/index.ts

@@ -6,6 +6,6 @@ export default class Dollar {
   }
 
   times(multiplier: number) {
-    this.amount = 5 * 2;
+    this.amount *= multiplier;
   }
 }