Implement a function classnames(...args) that conditionally joins CSS class names into a single string.

The function should:

• Accept any number of arguments.
• Handle strings, arrays, objects, and functions as input.
• Include a class name if the value is truthy.
• Recursively flatten arrays.
• Evaluate functions and include their returned value if truthy.
• Remove duplicate class names.

Example

classnames("btn", ["btn-primary", "active"], { disabled: false, hidden: true }, () => "dynamic");
// "btn btn-primary active hidden dynamic"

classnames("a", "b", "a", ["c", "b"]);
// "a b c"