[6.x] Add a more readable "missing" method to Filesystem and FilesystemAdapter

[AmirrezaNasiri]

This PR adds a new missing() method to both Illuminate\Filesystem\Filesystem and of course Illuminate\Filesystem\FilesystemAdapter so we can check missing files or directories in a more readable way.

In other word, instead of following condition checkings:

if (!Storage::exists('file.txt')) {
   // ...
}

if (!File::exists('file.txt')) {
   // ...
}

We can do:

if (Storage::missing('file.txt')) {
   // ...
}

if (File::missing('file.txt')) {
   // ...
}

There is no much logic behind the methods and I think they don't add maintenance overload.