Add alert modeling; upgrade to Selenium 4.33.0

Is this release, I added modeling of browser alerts. This resolves issue #213, introducing a framework feature that presents a more rational API, where alerts are treated as browser-level entities instead of the pseudo-context. The base class (AlertHandler) is an abstract class, with the methods that accept or dismiss the alert returning landing page objects. As shown in the unit tests, this base class should be implemented as a static class within the parent page model, and access to alert handler functions should be provided through page-class methods. The alert handler itself shouldn't be exposed for direct interaction, because this bypasses the management of page object within the framework.

I also upgraded Selenium 4 to the latest release (version 4.33.0).