[5.0] Add session helper functionality

...to bring it more in line with the `config` helper:

```php
session('key', 'default') // Session::get('key', 'default')

session(['key' => 'value']) // Session::set('key', 'value')

// Calling session() returns the store instance. Do with it whatever you want...

session()->forget('key') // Session::forget('key')
```
`$key === null` should always be preferred over an extra function call.