Add helper method to check whether ctrl+c was pressed, adopt it (#7482)

I've been working on streaming and pipeline interruption lately. It was
bothering me that checking ctrl+c (something we want to do often) always
requires a bunch of boilerplate like:
```rust
use std::sync::atomic::Ordering;

if let Some(ctrlc) = &engine_state.ctrlc {
     if ctrlc.load(Ordering::SeqCst) {
          ...
```
I added a helper method to cut that down to:

```rust
if nu_utils::ctrl_c::was_pressed(&engine_state.ctrlc) {
    ...
```

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

@@ -13,7 +13,7 @@ use std::collections::HashMap;
 use std::io::{BufRead, BufReader, Read, Write};
 use std::path::{Path, PathBuf};
 use std::process::{Command as CommandSys, Stdio};
-use std::sync::atomic::{AtomicBool, Ordering};
+use std::sync::atomic::AtomicBool;
 use std::sync::mpsc::{self, SyncSender};
 use std::sync::Arc;
 
@@ -745,10 +745,8 @@ fn read_and_redirect_message<R>(
         let length = bytes.len();
         buf_read.consume(length);
 
-        if let Some(ctrlc) = &ctrlc {
-            if ctrlc.load(Ordering::SeqCst) {
-                break;
-            }
+        if nu_utils::ctrl_c::was_pressed(&ctrlc) {
+            break;
         }
 
         match sender.send(bytes) {