`webpacker` should be a dev dependency

[DeedleFake]

Nothing in the actual implementation, as far as I can see, actually uses any webpacker-specific functionality, and I'm using this project just fine with vite-ruby. Unfortunately, adding the gem also adds webpacker itself as a dependency because this gem depends on it because it does use it in the tests. Would it be possible for webpacker to be marked as a dev dependency?