ci: move public-api goldens to goldens directory (#35768)

Moves the public api .d.ts files from tools/public_api_guard to
goldens/public-api.

Additionally, provides a README in the goldens directory and a script
assist in testing the current state of the repo against the goldens as
well as a command for accepting all changes to the goldens in a single
command.

PR Close #35768

goldens/public-api/common/testing/testing.d.ts

@@ -0,0 +1,64 @@
+export declare const MOCK_PLATFORM_LOCATION_CONFIG: InjectionToken<MockPlatformLocationConfig>;
+
+export declare class MockLocationStrategy extends LocationStrategy {
+    internalBaseHref: string;
+    internalPath: string;
+    internalTitle: string;
+    urlChanges: string[];
+    constructor();
+    back(): void;
+    forward(): void;
+    getBaseHref(): string;
+    getState(): unknown;
+    onPopState(fn: (value: any) => void): void;
+    path(includeHash?: boolean): string;
+    prepareExternalUrl(internal: string): string;
+    pushState(ctx: any, title: string, path: string, query: string): void;
+    replaceState(ctx: any, title: string, path: string, query: string): void;
+    simulatePopState(url: string): void;
+}
+
+export declare class MockPlatformLocation implements PlatformLocation {
+    get hash(): string;
+    get hostname(): string;
+    get href(): string;
+    get pathname(): string;
+    get port(): string;
+    get protocol(): string;
+    get search(): string;
+    get state(): unknown;
+    get url(): string;
+    constructor(config?: MockPlatformLocationConfig);
+    back(): void;
+    forward(): void;
+    getBaseHrefFromDOM(): string;
+    getState(): unknown;
+    onHashChange(fn: LocationChangeListener): void;
+    onPopState(fn: LocationChangeListener): void;
+    pushState(state: any, title: string, newUrl: string): void;
+    replaceState(state: any, title: string, newUrl: string): void;
+}
+
+export declare interface MockPlatformLocationConfig {
+    appBaseHref?: string;
+    startUrl?: string;
+}
+
+export declare class SpyLocation implements Location {
+    urlChanges: string[];
+    back(): void;
+    forward(): void;
+    getState(): unknown;
+    go(path: string, query?: string, state?: any): void;
+    isCurrentPathEqualTo(path: string, query?: string): boolean;
+    normalize(url: string): string;
+    onUrlChange(fn: (url: string, state: unknown) => void): void;
+    path(): string;
+    prepareExternalUrl(url: string): string;
+    replaceState(path: string, query?: string, state?: any): void;
+    setBaseHref(url: string): void;
+    setInitialPath(url: string): void;
+    simulateHashChange(pathname: string): void;
+    simulateUrlPop(pathname: string): void;
+    subscribe(onNext: (value: any) => void, onThrow?: ((error: any) => void) | null, onReturn?: (() => void) | null): SubscriptionLike;
+}