allow -h flags for export subcommands (#6189)

* allow `-h` flags for `export` subcommands

* remove unnecessary check

* add tests

* fmt

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

@@ -0,0 +1,15 @@
+use nu_test_support::{nu, pipeline};
+
+#[test]
+fn export_subcommands_help() {
+    let actual = nu!(
+        cwd: ".", pipeline(
+        r#"
+        export def -h
+        "#
+    ));
+
+    assert!(actual
+        .out
+        .contains("Define a custom command and export it from a module"));
+}