Fix rewrite shape in Storybook (#24827)

This fixes a bug in `@next/plugin-storybook` caused by an update in the rewrite structure, solution was provided by @stefanprobst https://github.com/vercel/next.js/issues/19345#issuecomment-818856655

Secondary question: it would be awesome if someone at Next could help translating `preset.js` into TypeScript, this would catch those bugs immediately. I've tried in https://github.com/vercel/next.js/pull/18367 but failed to build correctly.

## Bug

- [X] Related issues linked using `fixes #number` => parent issue is an umbrella of various small issues: https://github.com/vercel/next.js/issues/19345
- [X] Integration tests added => reproduction is implemented in https://github.com/lbke/next-plugin-storybook-demo until the package is more stable

packages/next-plugin-storybook/preset.js

@@ -15,7 +15,7 @@ async function webpackFinal(config) {
     target: 'server',
     config: nextConfig,
     buildId: 'storybook',
-    rewrites: [],
+    rewrites: { beforeFiles: [], afterFiles: [], fallback: [] },
   })
 
   config.plugins = [...config.plugins, ...nextWebpackConfig.plugins]