Add kanji filtering utility

lib/util/text_filtering.dart

@@ -26,3 +26,13 @@ const String rawHiraganaRegex = r'\p{Script=Hiragana}';
 final RegExp kanjiRegex = RegExp(rawKanjiRegex, unicode: true);
 final RegExp katakanaRegex = RegExp(rawKatakanaRegex, unicode: true);
 final RegExp hiraganaRegex = RegExp(rawHiraganaRegex, unicode: true);
+
+final kanjiPattern = RegExp(r'[\u3400-\u4DB5\u4E00-\u9FCB\uF900-\uFA6A]');
+List<String> filterKanjiSuggestions(String string) {
+  return kanjiPattern
+      .allMatches(string)
+      .map((match) => match.group(0))
+      .where((element) => element != null)
+      .map((element) => element!)
+      .toList();
+}