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/testability/testability.ts

@@ -138,6 +138,9 @@ export class TestabilityRegistry {
 /**
  * Adapter interface for retrieving the `Testability` service associated for a
  * particular context.
+ *
+ * @experimental Testability apis are primarily intended to be used by e2e test tool vendors like
+ * the Protractor team.
  */
 export interface GetTestability {
   addToWindow(registry: TestabilityRegistry): void;