fix(@angular-devkit/build-angular): show warning when using stylus

Stylus is not actively maintained and only 0.3% of the Angular CLI users use it.

Author: alan-agius4

packages/angular_devkit/build_angular/src/webpack/configs/styles.ts

@@ -98,6 +98,13 @@ export function getStylesConfig(wco: WebpackConfigOptions): webpack.Configuratio
     extraPlugins.push(new RemoveHashPlugin({ chunkNames: noInjectNames, hashFormat }));
   }
 
+  if (globalStylePaths.some(p => p.endsWith('.styl'))) {
+    wco.logger.warn(
+      'Stylus usage is deprecated and will be removed in a future major version. ' +
+      'To opt-out of the deprecated behaviour, please migrate to another stylesheet language.',
+    );
+  }
+
   let sassImplementation: {} | undefined;
   try {
     // tslint:disable-next-line:no-implicit-dependencies