debugger: Allow users to shutdown debug sessions while they're booting (#34362)

This solves problems where users couldn't shut down sessions while
locators or build tasks are running.

I renamed `debugger::Session::Mode` enum to `SessionState` to be more
clear when it's referenced in other crates. I also embedded the boot
task that is created in `SessionState::Building` variant. This allows
sessions to shut down all created threads in their boot process in a
clean and idiomatic way.

Finally, I added a method on terminal that allows killing the active
task.

Release Notes:

- Debugger: Allow shutting down debug sessions while they're booting up

crates/terminal/src/terminal.rs

@@ -1824,6 +1824,14 @@ impl Terminal {
         }
     }
 
+    pub fn kill_active_task(&mut self) {
+        if let Some(task) = self.task() {
+            if task.status == TaskStatus::Running {
+                self.pty_info.kill_current_process();
+            }
+        }
+    }
+
     pub fn task(&self) -> Option<&TaskState> {
         self.task.as_ref()
     }