Add experimental custom webpack support (#98862)
### What?
Add the experimental `customWebpack` configuration option so
webpack-mode applications can use a project-installed webpack version
instead of the version bundled with Next.js. Webpack is now an optional
`^5.98.0` peer dependency, and the new option is documented with
end-to-end coverage for webpack 5.98.0 and 5.111.0.
### Why?
Next.js currently redirects webpack imports to its bundled
implementation. This prevents applications from opting into newer
compatible webpack releases or validating upgrades independently. The
new option provides an explicit escape hatch while preserving the
bundled implementation as the default.
### How?
After configuration is resolved, webpack-mode startup redirects the
existing webpack require-hook aliases to the package resolved from the
application. The selection is propagated to webpack build workers
through the existing local-webpack runtime switch. If the optional peer
cannot be resolved, Next.js restores its bundled aliases and reports an
actionable installation error.
The implementation only activates for webpack, leaving Turbopack and
Rspack unchanged. It also handles the resolver stack API exposed by
external webpack versions. The test matrix verifies the selected
compiler version, rendering in development and production, the
build-worker path, missing-peer diagnostics, and bundled fallback
behavior when the option is disabled.
### Verification
- `pnpm test-dev-webpack
test/e2e/app-dir/custom-webpack/custom-webpack.test.ts`
- `pnpm test-start-webpack
test/e2e/app-dir/custom-webpack/custom-webpack.test.ts`
- `pnpm --filter=next types`
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Co-authored-by: vercel-fleet-prod[bot] <318278635+vercel-fleet-prod[bot]@users.noreply.github.com>
Co-authored-by: Tobias Koppers <1365881+sokra@users.noreply.github.com>
