Rewrite the plugin in a crossplatform way

Doing this will add Windows support (and will also work with shells other than `bash`) (issue #1). Just need to replace `subprocess.call` and instead use `Popen`, save the stdin of `flutter run` and send commands from there.

Look at the plugin code to find where `subprocess.call` is used for `flutter run`. Replace it with `Popen` to capture stdin. Test sending commands to the Flutter process. Ensure it works on Windows and with different shells.

Written by the indexing model from the issue text.