Preload inject_styles.js

[benmccann]

Originally reported by @ArekBartnik in #1524 (comment)

I'm really stumped on how to do this cleanly. See rollup/rollup#3790

I could do something kind of hacky and just look for a file named inject_styles.[hash].js. However, I've refactored the plugin to be two plugins now and would like to move out the css injection plugin to live in a separate codebase. That will give the user control over css handling and they can use the css code splitting and injection plugin, or another plugin to put everything in a single css bundle, or rollup-post-css or whatever else they want to do. At that point, it seems too tied to the current implementation to be looking specifically for inject_styles