Format a number as a currency string

Given a number, we can format it as a currency string without using an external libary.
The `NumberFormat` API provides the easy way to format a currency of a given country:
const formatter = new Intl.NumberFormat('en-US', {
style: 'currency',
currency: 'USD',
minimumFractionDigits: 2,
The optional `minimumFractionDigits` parameter indicates the minium number of digits in the fraction part. Calling the `format` function will format the input, and prefix or suffix the currency depending on the country.
formatter.format(2345); // '$2,345.00'
formatter.format('2345'); // '$2,345.00'
formatter.format('10000000'); // '$10,000,000.00'