[code-infra] Fix bundler tests and cover @mui/material-nextjs

[Janpot]

• Fix a regression in bundler tests caused by a change in pnpm pack output
• Add @mui/material-nextjs to the fixture. We've been seeing issues when moving to ESM exports with this package as it imports a cjs-only module from next. This improved coverage of this edge case while upgrading our package layout
• Add react@18 for next+webpack4 fixture. (We can probably think about deprecating support for webpack 4)

[mui-bot]

Netlify deploy preview

https://deploy-preview-44966--material-ui.netlify.app/

Bundle size report

No bundle size changes (Toolpad) No bundle size changes

Generated by :no_entry_sign: dangerJS against 25d05392344ebc099aaac3d7d0803149b30deb2d