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/security.ts

@@ -12,6 +12,8 @@
  * handled.
  *
  * See DomSanitizationService for more details on security in Angular applications.
+ *
+ * @stable
  */
 export enum SecurityContext {
   NONE,
@@ -25,6 +27,8 @@ export enum SecurityContext {
 /**
  * SanitizationService is used by the views to sanitize potentially dangerous values. This is a
  * private API, use code should only refer to DomSanitizationService.
+ *
+ * @stable
  */
 export abstract class SanitizationService {
   abstract sanitize(context: SecurityContext, value: string): string;