Explore the ways of making the plugin less webpack-dependent

[SevInf]

Right now plugin expects that webpack is used. Using other bundlers is not possible, and non-js imports need to be stubbed to avoid the errors inside express server.
One possible way to solve this is making bundler customizable.

Another one is removing bundling from project at all and expect tests to be built before gemini is executed. Runtime wrapping on client and server can be replaced by AOT compiltion in this case.

[apsavin]

It's cool that anyone who uses webpack can so easily use gemini-react now. Maybe, we can just move bundler into gemini-react-webpack-bundler?

So, if someone will want to use gemini-react with rollup, for example, it will be possible to write a plugin.