kana_transliteration: add test detecting incorrect ー chaining

src/kana_transliteration.rs

@@ -619,6 +619,11 @@ mod tests {
         );
     }
 
+    #[test]
+    fn test_hiragana_to_katakana_dont_chain_across_different_vowels() {
+        assert_eq!(transliterate_hiragana_to_katakana("ああい"), "アーイ");
+    }
+
     /// FULLWIDTH/HALFWIDTH
 
     #[test]