add a helpful msg to indicate a job has been frozen (#15206)

# Description
As stated in the title, when pressing ctrl-z, I sometimes feel confused
because I return to the REPL without any message. I don't know if the
process has been killed or suspended.

This PR aims to add a message to notify the user that the process has
been frozen.

# User-Facing Changes
After pressing `ctrl-z`.  A message will be printed in repl.


![图片](https://github.com/user-attachments/assets/5fe502eb-439e-4022-889f-64ba52cc2825)

# Tests + Formatting
NaN

# After Submitting
NaN

crates/nu-command/src/experimental/job_unfreeze.rs

@@ -144,6 +144,9 @@ fn unfreeze_job(
                     jobs.add_job_with_id(old_id, Job::Frozen(FrozenJob { unfreeze: handle }))
                         .expect("job was supposed to be removed");
 
+                    if state.is_interactive {
+                        println!("\nJob {} is re-frozen", old_id.get());
+                    }
                     Ok(())
                 }
 

crates/nu-command/src/system/run_external.rs

@@ -313,6 +313,7 @@ impl Command for External {
 
         let jobs = engine_state.jobs.clone();
         let this_job = engine_state.current_thread_job.clone();
+        let is_interactive = engine_state.is_interactive;
         let child_pid = child.pid();
 
         // Wrap the output into a `PipelineData::ByteStream`.
@@ -330,7 +331,10 @@ impl Command for External {
                 if let ForegroundWaitStatus::Frozen(unfreeze) = status {
                     let mut jobs = jobs.lock().expect("jobs lock is poisoned!");
 
-                    jobs.add_job(Job::Frozen(FrozenJob { unfreeze }));
+                    let job_id = jobs.add_job(Job::Frozen(FrozenJob { unfreeze }));
+                    if is_interactive {
+                        println!("\nJob {} is frozen", job_id.get());
+                    }
                 }
             }))),
         )?;