addToHistory method
Implementation
Future<void> addToHistory(String itemJson, int limit) async {
final prefs = await SharedPreferences.getInstance();
List<String> history = await getHistory();
history.add(itemJson);
history = history.toSet().toList();
if (history.length > limit) {
history = history.sublist(1);
}
await prefs.setStringList(_historyKey, history);
}