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/playground/src/key_events/index.ts

@@ -8,7 +8,8 @@
 
 import {bootstrap} from '@angular/platform-browser-dynamic';
 import {Component} from '@angular/core';
-import {KeyEventsPlugin} from '@angular/platform-browser';
+// TODO: remove deep import by reimplementing the event name serialization
+import {KeyEventsPlugin} from "@angular/platform-browser/src/dom/events/key_events";
 
 @Component({
   selector: 'key-events-app',