fix(@angular-devkit/build-angular): support mjs extension

fix for angular/angular-cli#10744

Author: amitport

Diff for: packages/angular_devkit/build_angular/src/angular-cli-files/models/webpack-configs/common.ts

@@ -237,7 +237,7 @@ export function getCommonConfig(wco: WebpackConfigOptions) {
     mode: buildOptions.optimization ? 'production' : 'development',
     devtool: false,
     resolve: {
-      extensions: ['.ts', '.js'],
+      extensions: ['.ts', '.mjs', '.js'],
       symlinks: !buildOptions.preserveSymlinks,
       modules: [
         wco.tsConfig.options.baseUrl || projectRoot,