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/http/src/http.ts

@@ -103,7 +103,8 @@ function mergeOptions(
  * http.get('request-from-mock-backend.json').subscribe((res:Response) => doSomething(res));
  * ```
  *
- **/
+ * @experimental
+ */
 @Injectable()
 export class Http {
   constructor(protected _backend: ConnectionBackend, protected _defaultOptions: RequestOptions) {}
@@ -186,6 +187,10 @@ export class Http {
   }
 }
 
+
+/**
+ * @experimental
+ */
 @Injectable()
 export class Jsonp extends Http {
   constructor(backend: ConnectionBackend, defaultOptions: RequestOptions) {