Add `map_using` to `starmap` and add more `.using_processes`/`.using_threads` methods

Should add a `map_using` keyword argument to `starmap` and use it to add `starmap.using_processes` and `starmap.using_threads`.

We should also probably set
```
map.using_processes = process_map
map.using_threads = thread_map
```
and make sure all of these properties are supported in the type stub.

We can also potentially add `.using_async` for these as well.