Fixes https://github.com/flutter/flutter/issues/130277

This PR does two things:

1. introduce a hidden `flutter build _preview` command, that will build a debug windows desktop app and copy it into the SDK's binary cache. This command is only intended to be run during packaging.
2. introduce a new device type, called `PreviewDevice`, which relies on the prebuilt desktop debug app from step 1, copies it into the target app's assets build folder, and then hot reloads their dart code into it.