add -e flag to print, to print the value to stderr (#5935)

* Refactor: make stdout write all and flush as generic function

* support print to stderr

crates/nu-command/tests/commands/print.rs

@@ -0,0 +1,23 @@
+use nu_test_support::{nu, pipeline};
+
+#[test]
+fn print_to_stdout() {
+    let actual = nu!(
+        cwd: ".", pipeline(
+            "print 'hello world'"
+        )
+    );
+    assert!(actual.out.contains("hello world"));
+    assert!(actual.err.is_empty());
+}
+
+#[test]
+fn print_to_stderr() {
+    let actual = nu!(
+        cwd: ".", pipeline(
+            "print -e 'hello world'"
+        )
+    );
+    assert!(actual.out.is_empty());
+    assert!(actual.err.contains("hello world"));
+}