Introducing useful event when a debug session has been cleared, so that all subscribers can unsubscribe from any cleared session automatically

[adri09070]

When a debug session is cleared, its process becomes nil so we can't get information from this process anymore.
Also, it means any subscribed debugger shouldn't be able to perform any action on the debug session anymore. That's why, a debugger that has a cleared debug session should unsubscribe from the debug session and from the SystemAnnouncer (it is subscribed to the SystemAnnouncer to get notified when a method has been compiled/recompiled).

For now, we don't have a mechanism that does that automatically. All debuggers should call their method #clear, which is annoying, especially in tests. Because if someone instanciates a debugger in a test but doesn't know that they should ensure the method #clear is called, then some errors (like this one: pharo-spec/NewTools#579) might happen because the debugger executes code due to subscriptions that should have been cancelled, causing some messages to be sent to a nil process.

This PR introduces an event triggered when a session is cleared so that all subscribers can unsubscribe from whatever they should unsubscribe from

[jecisc]

About the CI: If tonel tests are broken it's not related. It's because we need to update Tonel version in Iceberg (that loads the tests) and Pharo (that loads the core) and for now only iceberg was updated and it breaks the new tonel tests.

[jecisc]

Old failing tests are related to Tonel so this is ok to merge