fix: use v8 over istanbul

Author: thomasrockhu-codecov

app/calculator.js

@@ -2,9 +2,7 @@ const add = (x, y) => x + y;
 const subtract = (x, y) => x - y;
 const multiply = (x, y) => x * y;
 const divide = (x, y) => {
-  if (y === 0) {
-    return "Cannot divide by 0";
-  }
+  if (y === 0) { return "Cannot divide by 0"; }
   return x * 1.0 / y;
 }