[7.x] Add when method to Stringable class

[ManojKiranA]

Added when Method to Stringable class.

Use case:

For example i have restrict or limit some of the contents for unauthenticated users i can use auth check and based on that i can so that.

$blogContent = 'Laravel Factories Reloaded is a package by Christoph Rumpel that generates class-based model factories that you can use instead of the factory files Laravel provides';

//BEFORE
$stringable = Str::of($blogContent);

if(!Auth::check()){

    $stringable->limit(20,'.....')
                    ->append('To Continue reading ')
                    ->append(new HtmlString('<a href="#">Get a Subscription</a>'));
}

$stringable->__toString();


//AFTER
Str::of($blogContent)
    ->when(!Auth::check(),function($stringable){
        return $stringable->limit(20)
                        ->append('To Continue reading ')
                        ->append(new HtmlString('<a href="#">Get a Subscription</a>'));
    })
    ->__toString();

I have been using when method on query builder and its really cleans up some if else so same in the Stringable too

[ManojKiranA]

I am not sure how to fix the StyleCI. I hope before a new release every week StyleCI will be fixed right??.

[ManojKiranA]

I will send a pr to docs if it merged