feat: Add "!" command to `rustlings watch`

src/main.rs

@@ -301,10 +301,18 @@ fn spawn_watch_shell(
                     println!("  hint   - prints the current exercise's hint");
                     println!("  clear  - clears the screen");
                     println!("  quit   - quits watch mode");
+                    println!("  !<cmd> - executes a command, like `!rustc --explain E0381`");
                     println!("  help   - displays this help message");
                     println!();
                     println!("Watch mode automatically re-evaluates the current exercise");
                     println!("when you edit a file's contents.")
+                } else if let Some(cmd) = input.strip_prefix('!') {
+                    let parts: Vec<&str> = cmd.split_whitespace().collect();
+                    if parts.is_empty() {
+                        println!("no command provided");
+                    } else if let Err(e) = Command::new(parts[0]).args(&parts[1..]).status() {
+                        println!("failed to execute command `{}`: {}", cmd, e);
+                    }
                 } else {
                     println!("unknown command: {input}");
                 }