feat(service-worker): introduce the @angular/service-worker package (#19274)

This service worker is a conceptual derivative of the existing @angular/service-worker maintained at github.com/angular/mobile-toolkit, but has been rewritten to support use across a much wider variety of applications.

Entrypoints include:

@angular/service-worker: a library for use within Angular client apps to communicate with the service worker.
@angular/service-worker/gen: a library for generating ngsw.json files from glob-based SW config files.
@angular/service-worker/ngsw-worker.js: the bundled service worker script itself.
@angular/service-worker/ngsw-cli.js: a CLI tool for generating ngsw.json files from glob-based SW config files.

packages/service-worker/cli/main.ts

@@ -0,0 +1,29 @@
+/**
+ * @license
+ * Copyright Google Inc. All Rights Reserved.
+ *
+ * Use of this source code is governed by an MIT-style license that can be
+ * found in the LICENSE file at https://angular.io/license
+ */
+
+const {Generator, NgswConfig} = require('@angular/service-worker/config');
+const fs = require('fs');
+const path = require('path');
+import {NodeFilesystem} from './filesystem';
+
+
+const cwd = process.cwd();
+
+const distDir = path.join(cwd, process.argv[2]);
+const config = path.join(cwd, process.argv[3]);
+const baseHref = process.argv[4] || '/';
+
+const configParsed = JSON.parse(fs.readFileSync(config).toString());
+
+const filesystem = new NodeFilesystem(distDir);
+const gen = new Generator(filesystem, baseHref);
+
+(async() => {
+  const control = await gen.process(configParsed);
+  await filesystem.write('/ngsw.json', JSON.stringify(control, null, 2));
+})();