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/common/src/forms-deprecated/directives/validators.ts

@@ -30,6 +30,8 @@ import {NG_VALIDATORS, Validators} from '../validators';
  *   }
  * }
  * ```
+ *
+ * @experimental
  */
 export interface Validator { validate(c: AbstractControl): {[key: string]: any}; }
 

modules/@angular/common/src/location/platform_location.ts

@@ -53,8 +53,11 @@ export abstract class PlatformLocation {
 /**
  * A serializable version of the event from onPopState or onHashChange
  *
- * @stable
+ * @experimental
  */
 export interface UrlChangeEvent { type: string; }
 
+/**
+ * @experimental
+ */
 export interface UrlChangeListener { (e: UrlChangeEvent): any; }