refactor: split currency and key/metal formatting into separate units

this simplifies functions and tests, and will make currency optional/variable later

src/content/utils/currency.ts

@@ -0,0 +1,8 @@
+import { conversion_ref_usd } from '../config';
+
+export function convertTF2PriceToUSD(keys: number, metal: number, keyPrice: number): number {
+	const pureMetal = (keys * keyPrice) + metal;
+
+	// Round price up to nearest cent
+  return Math.ceil(pureMetal * conversion_ref_usd * 100) / 100
+}