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/src/interfaces.ts

@@ -11,6 +11,8 @@ import {ActivatedRouteSnapshot, RouterStateSnapshot} from './router_state';
 
 /**
  * An interface a class can implement to be a guard deciding if a route can be activated.
+ *
+ * @experimental
  */
 export interface CanActivate {
   canActivate(route: ActivatedRouteSnapshot, state: RouterStateSnapshot):
@@ -19,12 +21,17 @@ export interface CanActivate {
 
 /**
  * An interface a class can implement to be a guard deciding if a route can be deactivated.
+ *
+ * @experimental
  */
 export interface CanDeactivate<T> {
   canDeactivate(component: T, route: ActivatedRouteSnapshot, state: RouterStateSnapshot):
       Observable<boolean>|boolean;
 }
 
+/**
+ * @experimental
+ */
 export interface Resolve<T> {
   resolve(route: ActivatedRouteSnapshot, state: RouterStateSnapshot): Observable<any>|any;
-}
+}