Simplify seq char (#7054)

* Simplify `seq char`

* Fix input/output tests

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

@@ -0,0 +1,25 @@
+use nu_test_support::{nu, pipeline};
+
+#[test]
+fn fails_when_first_arg_is_multiple_chars() {
+    let actual = nu!(
+        cwd: ".", pipeline(
+        r#"
+        seq char aa z
+        "#
+    ));
+
+    assert!(actual.err.contains("should be 1 character long"));
+}
+
+#[test]
+fn fails_when_second_arg_is_multiple_chars() {
+    let actual = nu!(
+        cwd: ".", pipeline(
+        r#"
+        seq char a zz
+        "#
+    ));
+
+    assert!(actual.err.contains("should be 1 character long"));
+}