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.

tools/public_api_guard/service-worker/config.d.ts

@@ -0,0 +1,52 @@
+/** @experimental */
+export interface AssetGroup {
+    installMode?: 'prefetch' | 'lazy';
+    name: string;
+    resources: {
+        files?: Glob[];
+        versionedFiles?: Glob[];
+        urls?: Glob[];
+    };
+    updateMode?: 'prefetch' | 'lazy';
+}
+
+/** @experimental */
+export interface Config {
+    appData?: {};
+    assetGroups?: AssetGroup[];
+    dataGroups?: DataGroup[];
+    index: string;
+}
+
+/** @experimental */
+export interface DataGroup {
+    cacheConfig: {
+        maxSize: number;
+        maxAge: Duration;
+        timeout?: Duration;
+        strategy?: 'freshness' | 'performance';
+    };
+    name: string;
+    urls: Glob[];
+    version?: number;
+}
+
+/** @experimental */
+export declare type Duration = string;
+
+/** @experimental */
+export interface Filesystem {
+    list(dir: string): Promise<string[]>;
+    read(file: string): Promise<string>;
+    write(file: string, contents: string): Promise<void>;
+}
+
+/** @experimental */
+export declare class Generator {
+    readonly fs: Filesystem;
+    constructor(fs: Filesystem, baseHref: string);
+    process(config: Config): Promise<Object>;
+}
+
+/** @experimental */
+export declare type Glob = string;