Parser panic for signature with multibyte char for short flag #8821 (#8849)

Hey I'm a developer and I'm still new to nushell and rust but I would
like to learn more about both. This is my first PR for this project.

The intent of my change is to allow to use multibyte utf-8 characters in
commands short flags.

tests/parsing/mod.rs

@@ -237,3 +237,14 @@ fn numberlike_command_name() {
 
     assert!(actual.err.contains("backticks"));
 }
+
+#[test]
+fn call_command_with_non_ascii_argument() {
+    let actual = nu!(cwd: "tests/parsing/samples",
+        r#"
+            def nu-arg [--umlaut(-ö): int] {}
+            nu-arg -ö 42
+        "#);
+
+    assert_eq!(actual.err.len(), 0);
+}