Add web.session.Persistence base class superseding the ISession interface

[thekid]

use web\session\InFileSystem;

$sessions= new InFileSystem()->lasting(3600);
$session= $sessions->create();

$session->expires(); // time() + 3600

⚠️ This PR doesn't touch the ISession interface for obvious BC problems! Instead, it introduced a new web.session.Persistence base class with this method readily implemented. It supersedes the interface while implementing it for the time being.

[thekid]

The web.Session base class has the following declaration:

public abstract class web.Session {
  public abstract function id(): string
  public abstract function expires(): int
  public abstract function register(string $name, var $value): void
  public abstract function value(string $name, var $default): var
  public abstract function remove(string $name): void
  public abstract function destroy(): void
}

Maybe this method should be called expires() instead?

[thekid]

Released in https://github.com/xp-forge/sessions/releases/tag/v4.0.0