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/backends/xhr_backend.ts

@@ -31,6 +31,8 @@ const XSSI_PREFIX = /^\)\]\}',?\n/;
  *
  * This class would typically not be created or interacted with directly inside applications, though
  * the {@link MockConnection} may be interacted with in tests.
+ *
+ * @experimental
  */
 export class XHRConnection implements Connection {
   request: Request;
@@ -156,6 +158,8 @@ export class XHRConnection implements Connection {
  * Applications can configure custom cookie and header names by binding an instance of this class
  * with different `cookieName` and `headerName` values. See the main HTTP documentation for more
  * details.
+ *
+ * @experimental
  */
 export class CookieXSRFStrategy implements XSRFStrategy {
   constructor(
@@ -193,7 +197,8 @@ export class CookieXSRFStrategy implements XSRFStrategy {
  *   }
  * }
  * ```
- **/
+ * @experimental
+ */
 @Injectable()
 export class XHRBackend implements ConnectionBackend {
   constructor(