fix: public api surface fixes + stability markers

- ts-api-guardian will now error if a new public symbol is added with a stability marker (`@stable`, `@experimental`, `@deprecated`)
- DomEventsPlugin and KeyEventsPlugin were removed from public api surface - these classes is an implementation detail
- deprecated BROWSER_PROVIDERS was removed completely
- `@angular/compiler` was removed from the ts-api-guardian check since this package shouldn't contain anything that users need to directly import
- the rest of the api surface was conservatively marked as stable or experimental

BREAKING CHANGES: DomEventsPlugin and KeyEventsPlugin previously exported from core are no longer public - these classes are implementation detail.

Previously deprecated BROWSER_PROVIDERS was completely removed from platform-browser.

Closes #9236
Closes #9235
Ref #9234

modules/@angular/router/index.ts

@@ -20,4 +20,8 @@ export {ActivatedRoute, ActivatedRouteSnapshot, RouterState, RouterStateSnapshot
 export {PRIMARY_OUTLET, Params} from './src/shared';
 export {DefaultUrlSerializer, UrlPathWithParams, UrlSerializer, UrlTree} from './src/url_tree';
 
-export const ROUTER_DIRECTIVES = [RouterOutlet, RouterLink, RouterLinkActive];
+
+/**
+ * @experimental
+ */
+export const ROUTER_DIRECTIVES = [RouterOutlet, RouterLink, RouterLinkActive];