[12.x] Allow any \Stringable implementation for Illuminate\Support\Stringable::exactly()

[Rahul-Mac]

Currently, Illuminate\Support\Stringable::exactly() only supports string or an instance of Illuminate\Support\Stringable as the $value parameter.

This PR aims to enhance the allowed types for exactly() by accepting any object that implements \Stringable.

Let's assume we have an object like this:

$object = new class () implements \Stringable {
    public function __toString(): string
    {
        return "foo";
    }
}

Earlier:

Str::of('foo')->exactly($object); // false

Now:

Str::of('foo')->exactly($object); // true

[taylorotwell]

I think it's fine as is.