classnames Utility

Implement classNames(...args), a utility that builds a CSS class string, like the popular classnames package.

Each argument can be:

• a string: included if it is not empty,
• a number: included (as text) if it is not 0 and not NaN,
• an object: each key is included if its value is truthy,
• an array: processed recursively with the same rules (nested arrays too),
• anything else falsy (false, null, undefined): ignored.

The result joins the included names with a single space, in order. There is no leading or trailing space and no de-duplication. If nothing is included, return "".