fix(webpack): bump enhanced-resolve to 5.18.4 for #/ subpath imports

[controversial]

[NOTE: claude-authored, i have verified the tests & reviewed the diff]

What?

Follow-up to #94461 to enable the identical behavior in Webpack by bumping enhanced-resolve by one patch version (5.18.3 → 5.18.4)

Fixes Webpack failing to resolve imports using the common "#/*": "./src/*" pattern in package.json imports field.

Bumps the vendored enhanced-resolve to 5.18.4 (via pnpm.overrides, alongside the existing webpack pin) and recompiles packages/next/src/compiled/webpack/bundle5.js. Adds an e2e test covering "imports": { "#/*": "./src/*" } for both the app package and an external package.

Why?

Brings resolution behavior to parity with

• current LTS node versions (module: allow subpath imports that start with #/ nodejs/node#60864)
• as well as TypeScript 6 (Allow subpath imports that start with #/ microsoft/TypeScript#62844)
• and now Turbopack (fix(turbopack): allow #/ prefixed subpath import specifiers #94461).

enhanced-resolve picked this up in webpack/enhanced-resolve#468 (v5.18.4), but Next.js still vendors 5.18.3, so next dev --webpack / next build --webpack reject import x from "#/foo" with Package import specifier "#/foo" is not defined / should not start with "#/".

How?

• Add "enhanced-resolve": "5.18.4" to pnpm.overrides (webpack@5.98.0 depends on ^5.17.1, so this stays in-range)
• Regenerate bundle5.js via taskr ncc_webpack_bundle5 + ncc_webpack_bundle_packages
• New test/e2e/app-dir/subpath-imports-slash fixture:
  • App package.json with "imports": { "#/*": "./src/*" }, page imports #/greeting
  • node_modules/external-slash-pkg with its own "#/*": "./lib/*", forced through the bundler via transpilePackages
  • typescript.ignoreBuildErrors: true so the auto-generated tsconfig (no paths) does not add a JsConfigPathsPlugin alias that would mask the imports-field resolution under test

Follow-up to #94461
Refs #94290