search: add function for fetching multiple kanji at once

lib/search.dart

@@ -19,6 +19,10 @@ extension JaDBConnection on DatabaseExecutor {
   Future<KanjiSearchResult?> jadbSearchKanji(String kanji) =>
       searchKanjiWithDbConnection(this, kanji);
 
+  /// Search for a kanji in the database.
+  Future<Map<String, KanjiSearchResult>> jadbGetManyKanji(Set<String> kanji) =>
+      searchManyKanjiWithDbConnection(this, kanji);
+
   /// Filter a list of characters, and return the ones that are listed in the kanji dictionary.
   Future<List<String>> filterKanji(
     List<String> kanji, {

lib/search/kanji_search.dart

@@ -219,3 +219,25 @@ Future<KanjiSearchResult?> searchKanjiWithDbConnection(
     dictionaryReferences: dictionaryReferences,
   );
 }
+
+// TODO: Use fewer queries with `IN` clauses to reduce the number of queries
+
+Future<Map<String, KanjiSearchResult>> searchManyKanjiWithDbConnection(
+  DatabaseExecutor connection,
+  Set<String> kanji,
+) async {
+  if (kanji.isEmpty) {
+    return {};
+  }
+
+  final results = <String, KanjiSearchResult>{};
+
+  for (final k in kanji) {
+    final result = await searchKanjiWithDbConnection(connection, k);
+    if (result != null) {
+      results[k] = result;
+    }
+  }
+
+  return results;
+}