Add support for objects with `toString` methods

Hi there, I'm using a css-in-js library (stitches) which returns class names as objects with custom toString methods. This requires calling toString before passing the "class names" to clsx- otherwise clsx assumes the object is a config/mapping object.

// Current usage
const pink = css({ color: "pink" });
clsx(pink().toString())

I currently have a patch package which adds an additional check to the toVal function, checking to see if a mix object has it's own toString method and calls/returns that if so.

// General idea
if (mix.hasOwnProperty("toString")) {
  str += mix.toString();
}

I noticed that classnames has a condition for this (original PR). React also calls toString if you pass an object to className. I'm wondering if it might be useful to include this behavior in clsx? I can contribute a PR if so.