feat(@angular-devkit/build-angular): allow transformation of i18n extraction builder configuration

This change allows the underlying webpack configuration of the i18n extraction builder to be modified when programmatically using the builder.
Programmatic usage of the `@angular-devkit/build-angular` builders is currently considered experimental.

Author: clydin

packages/angular_devkit/build_angular/src/extract-i18n/index.ts

@@ -25,6 +25,7 @@ import {
 } from '../angular-cli-files/models/webpack-configs';
 import { statsErrorsToString, statsHasErrors, statsHasWarnings, statsWarningsToString } from '../angular-cli-files/utilities/stats';
 import { Schema as BrowserBuilderOptions } from '../browser/schema';
+import { ExecutionTransformer } from '../transforms';
 import { createI18nOptions } from '../utils/i18n-options';
 import { assertCompatibleAngularVersion } from '../utils/version';
 import { generateBrowserWebpackConfigFromContext } from '../utils/webpack-browser-config';
@@ -83,6 +84,9 @@ class InMemoryOutputPlugin {
 export async function execute(
   options: ExtractI18nBuilderOptions,
   context: BuilderContext,
+  transforms?: {
+    webpackConfiguration?: ExecutionTransformer<webpack.Configuration>;
+  },
 ): Promise<BuildResult> {
   // Check Angular version.
   assertCompatibleAngularVersion(context.workspaceRoot, context.logger);
@@ -226,10 +230,14 @@ export async function execute(
     }
   };
 
-  const webpackResult = await runWebpack(config, context, {
-    logging,
-    webpackFactory: await import('webpack'),
-  }).toPromise();
+  const webpackResult = await runWebpack(
+    (await transforms?.webpackConfiguration?.(config)) || config,
+    context,
+    {
+      logging,
+      webpackFactory: await import('webpack'),
+    },
+  ).toPromise();
 
   // Complete if using VE
   if (!usingIvy) {