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.

Do mô hình lập chỉ mục viết ra từ nội dung của issue.