Dynamically switch to `react-server-dom-webpack` when available in user project `package.json` instead of bundled `@vitejs/plugin-rsc/vendor/react-server-dom`

Currently `@vitejs/plugin-rsc` bundles `react-server-dom-webpack` inside its own `vendor/react-server-dom` directory.

This is a feature request to allow users to install its own version of `react-server-dom-webpack` and use that internally instead of `@vitejs/plugin-rsc/vendor/react-server-dom`.
