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/forms/src/form_providers.ts

@@ -18,7 +18,7 @@ import {FormBuilder as NewFormBuilder} from './form_builder';
 
 
 
-/*
+/**
  * Shorthand set of providers used for building Angular forms.
  *
  * ### Example
@@ -43,6 +43,10 @@ function flatten(platformDirectives: any[]): any[] {
   return flattenedDirectives;
 }
 
+
+/**
+ * @experimental
+ */
 export function disableDeprecatedForms(): any[] {
   return [{
     provide: CompilerConfig,
@@ -55,6 +59,9 @@ export function disableDeprecatedForms(): any[] {
   }];
 }
 
+/**
+ * @experimental
+ */
 export function provideForms(): any[] {
   return [
     {provide: PLATFORM_DIRECTIVES, useValue: NEW_FORM_DIRECTIVES, multi: true}, FORM_PROVIDERS