whereStartsWith method
Add a constraint for finding objects where the String value in column
starts with prefix
Implementation
void whereStartsWith(
  String column,
  String prefix, {
  bool caseSensitive = false,
}) {
  prefix = Uri.encodeComponent(prefix);
  if (caseSensitive) {
    queries.add(MapEntry<String, dynamic>(
        _singleQuery, '"$column":{"\$regex": "^$prefix"}'));
  } else {
    queries.add(MapEntry<String, dynamic>(
        _singleQuery, '"$column":{"\$regex": "^$prefix", "\$options": "i"}'));
  }
}