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/src/util/decorators.ts

@@ -12,6 +12,8 @@ var _nextClassId = 0;
 
 /**
  * Declares the interface to be used with {@link Class}.
+ *
+ * @stable
  */
 export interface ClassDefinition {
   /**
@@ -58,6 +60,7 @@ export interface ClassDefinition {
  * @ng.View({...})
  * class MyClass {...}
  * ```
+ * @stable
  */
 export interface TypeDecorator {
   /**