[8.x] Add more readable missing method to Session\Store

[schmidfelix]

This PR adds a new missing() method to Illuminate\Session\Store so we can check for missing keys in a more readable way.

// Instead of
if (!Session::has('key') {
    // ...
}

// We can do
if (Session::missing('key') {
    // ...
}

Like the missing method on the filesystem, there is no much logic behind the method, and it shouldn't add too much maintenance overload.