delete method
Implementation
Future<T?> delete(T t)async{
final jsn=config.toDocWithId(t);
try{
await fbDoc(jsn[idKey]).delete();
}on FirebaseException catch(e){
config.onException(e);
return null;
}
final i=(state.value??[]).indexWhere((e) => getId(e)==getId(t));
if(i!=-1)
state=state.whenData((v) => List.of(v..removeAt(i)));
return t;
}