Add utility belt example

examples/utility-belt/src/string-to-number.test.js

@@ -0,0 +1,15 @@
+import { describe, it, expect } from 'vitest';
+import { stringToNumber } from './string-to-number';
+
+describe('stringToNumber', () => {
+  it('converts a string to a number', () => {
+    expect(stringToNumber('42')).toBe(42);
+  });
+
+  it('throws an error if given a string that is not a number', () => {
+    const value = 'foo';
+    expect(() => stringToNumber(value)).toThrowError(
+      `cannot be parsed as a number`,
+    );
+  });
+});