fix: crash in custom importers with worker threads (#958)

Author: alexander-akait

src/utils.js

@@ -81,9 +81,9 @@ function proxyCustomImporters(importers, loaderContext) {
  return [].concat(importers).map(
  (importer) =>
  function proxyImporter(...args) {
- this.webpackLoaderContext = loaderContext;
+ const self = { ...this, webpackLoaderContext: loaderContext };
 
- return importer.apply(this, args);
+ return importer.apply(self, args);
  }
  );
 }