Make example binaries proper cargo examples (#7019)

Should not be built by default with `cargo build`
Instead are compiled with `cargo test` to avoid bitrot
Run with `cargo run -p ... --example ...`

crates/nu-system/examples/demo.rs

@@ -0,0 +1,32 @@
+fn main() {
+    #[cfg(any(
+        target_os = "android",
+        target_os = "linux",
+        target_os = "macos",
+        target_os = "windows"
+    ))]
+    {
+        let cores = match std::thread::available_parallelism() {
+            Ok(p) => p.get(),
+            Err(_) => 1usize,
+        };
+        for run in 1..=10 {
+            for proc in nu_system::collect_proc(std::time::Duration::from_millis(100), false) {
+                if proc.cpu_usage() > 0.00001 {
+                    println!(
+                        "{} - {} - {} - {} - {:.2}% - {}M - {}M - {} procs",
+                        run,
+                        proc.pid(),
+                        proc.name(),
+                        proc.status(),
+                        proc.cpu_usage() / cores as f64,
+                        proc.mem_size() / (1024 * 1024),
+                        proc.virtual_size() / (1024 * 1024),
+                        cores,
+                    )
+                }
+            }
+            std::thread::sleep(std::time::Duration::from_millis(1000));
+        }
+    }
+}