[8.x] Add unless method on stringable

[PH7-Jack]

This pr aims to increase the code reading by placing the unless function to perform some callback.

Example of use:

$authenticated = false;

Str::of('Hello')
  ->unless($authenticated, function($stringable) {
    return $stringable->append(' Guest!');
    
  })
  ->when($authenticated, function($stringable) {
    return $stringable->append(' ' . auth()->user()->name);
  });

[selcukcukur]

Str::of('Hello')->when($authenticated, function($stringable) {
      return $authenticated 
           ? $stringable->append(' ' . auth()->user()->name)
           : $stringable->append(' Guest!');
});

Why don't you prefer to use it this way?

[PH7-Jack]

@selcukcukur I also need to use only unless, with no other conditions to apply css classes using blade class component

Ex:

Str::of('px-4 py-2 bg-white')
  ->unless($borderless, function($stringable) {
    return $stringable->append(' border border-gray-300!');
  })

That way it is less verbose than using the when method

Str::of('px-4 py-2 bg-white')
  ->when($borderless === false, function($stringable) {
    return $stringable->append(' border border-gray-300!');
  })