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/http/src/enums.ts

@@ -8,6 +8,7 @@
 
 /**
  * Supported http methods.
+ * @experimental
  */
 export enum RequestMethod {
   Get,
@@ -23,6 +24,7 @@ export enum RequestMethod {
  * All possible states in which a connection can be, based on
  * [States](http://www.w3.org/TR/XMLHttpRequest/#states) from the `XMLHttpRequest` spec, but with an
  * additional "CANCELLED" state.
+ * @experimental
  */
 export enum ReadyState {
   Unsent,
@@ -36,6 +38,7 @@ export enum ReadyState {
 /**
  * Acceptable response types to be associated with a {@link Response}, based on
  * [ResponseType](https://fetch.spec.whatwg.org/#responsetype) from the Fetch spec.
+ * @experimental
  */
 export enum ResponseType {
   Basic,
@@ -47,6 +50,7 @@ export enum ResponseType {
 
 /**
  * Supported content type to be automatically associated with a {@link Request}.
+ * @experimental
  */
 export enum ContentType {
   NONE,