feat: support webpack processContent hook for CSS embedded in JS

- Add `compilation.hooks.processContent` support to minimize CSS content
  embedded in JS bundles (text/style/css-style-sheet export types)
- Extract `transformSource` method to share minification logic, error
  handling, source map validation, and warning filtering between
  processAssets and processContent paths
- Extract `createWorkerFactory` for reusable lazy worker pool management

Author: xiaoxiaojx

package-lock.json

@@ -104,7 +104,7 @@
     "standard-version": "^9.5.0",
     "sugarss": "^5.0.0",
     "typescript": "^5.5.4",
-    "webpack": "^5.93.0"
+    "webpack": "https://pkg.pr.new/webpack@11c12b8"
   },
   "peerDependencies": {
     "webpack": "^5.0.0"