upsertAt method
Insert or replace a key/value pair in a Map.
Implementation
Map<K, V> Function(K key, V value) upsertAt(Eq<K> eq) => (K key, V value) {
final newMap = {...this};
newMap.addEntries([MapEntry(key, value)]);
return newMap.modifyAtIfPresent(eq)(key, (_) => value);
};