This PR allows using the Session attribute in functional API components.

Old way:

state('tab')->attribute(\Livewire\Attributes\Session::class, key: 'tab')

New way:

state('tab')->session(key: 'tab')

Notes:

• I added a minimum of tests where I deemed necessary by mimicking the ones testing the Url attributes.
• I believe a "default" argument would be awesome to add so I will make a PR to the Livewire repo before continuing here.