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/test/linker/security_integration_spec.ts

@@ -15,8 +15,8 @@ import {CompilerConfig} from '@angular/compiler';
 import {Component, ViewMetadata} from '@angular/core/src/metadata';
 import {IS_DART} from '../../src/facade/lang';
 import {el} from '@angular/platform-browser/testing/browser_util';
+import {DomSanitizationService} from '@angular/platform-browser/src/security/dom_sanitization_service';
 
-import {DomSanitizationService} from '@angular/platform-browser';
 
 const ANCHOR_ELEMENT = /*@ts2dart_const*/ new OpaqueToken('AnchorElement');