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/platform-browser/testing/browser.ts

@@ -44,12 +44,16 @@ function createNgZone(): NgZone {
 
 /**
  * Default platform providers for testing.
+ *
+ * @stable
  */
 export const TEST_BROWSER_PLATFORM_PROVIDERS: Array<any /*Type | Provider | any[]*/> =
     TEST_BROWSER_STATIC_PLATFORM_PROVIDERS;
 
 /**
  * Default application providers for testing without a compiler.
+ *
+ * @stable
  */
 export const TEST_BROWSER_APPLICATION_PROVIDERS: Array<any /*Type | Provider | any[]*/> =
     [BROWSER_APP_PROVIDERS, ADDITIONAL_TEST_BROWSER_STATIC_PROVIDERS];