Add authentication assertions to `KernelBrowser`

[kbond]

use Zenstruck\Browser\Component\Auth;

$this->browser()
    ->visit('/some/page')

    // fail if authenticated
    ->assertNotAuthenticated()

    ->actingAs($user)

    // fail if NOT authenticated
    ->assertAuthenticated()

    // fail if NOT authenticated or authenticated as a user other than "kevin"
    ->assertAuthenticated('kevin')
;