Add utility belt example

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

@@ -0,0 +1,14 @@
+/**
+ * Converts a string to a number or throws an error if the string is not a number.
+ * @param {string} value
+ * @returns {number} The number.
+ */
+export const stringToNumber = (value) => {
+  const number = Number(value);
+
+  if (isNaN(number)) {
+    throw new Error(`'${value}' cannot be parsed as a number.`);
+  }
+
+  return number;
+};