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/platform-browser/testing/testing.d.ts

@@ -0,0 +1,4 @@
+export declare class BrowserTestingModule {
+}
+
+export declare const platformBrowserTesting: (extraProviders?: StaticProvider[] | undefined) => PlatformRef;