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/core/src/di/forward_ref.ts

@@ -15,6 +15,7 @@ import {Type, isFunction, stringify} from '../facade/lang';
  * ### Example
  *
  * {@example core/di/ts/forward_ref/forward_ref.ts region='forward_ref_fn'}
+ * @experimental
  */
 export interface ForwardRefFn { (): any; }
 

modules/@angular/core/src/di/reflective_injector.ts

@@ -355,6 +355,8 @@ export class ReflectiveInjectorDynamicStrategy implements ReflectiveInjectorStra
  *
  * Notice, we don't use the `new` operator because we explicitly want to have the `Injector`
  * resolve all of the object's dependencies automatically.
+ *
+ * @stable
  */
 export abstract class ReflectiveInjector implements Injector {
   /**

modules/@angular/core/src/di/reflective_provider.ts

@@ -47,6 +47,8 @@ const _EMPTY_LIST: any[] /** TODO #9100 */ = /*@ts2dart_const*/[];
  *
  * expect(injector.get('message')).toEqual('Hello');
  * ```
+ *
+ * @experimental
  */
 export interface ResolvedReflectiveProvider {
   /**