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/router/src/url_tree.ts

@@ -62,6 +62,8 @@ function containsSegmentHelper(
 
 /**
  * A URL in the tree form.
+ *
+ * @experimental
  */
 export class UrlTree {
   /**
@@ -96,6 +98,10 @@ export class UrlSegment {
   toString(): string { return serializePaths(this); }
 }
 
+
+/**
+ * @experimental
+ */
 export class UrlPathWithParams {
   constructor(public path: string, public parameters: {[key: string]: string}) {}
   toString(): string { return serializePath(this); }
@@ -153,6 +159,8 @@ export function mapChildrenIntoArray<T>(
 
 /**
  * Defines a way to serialize/deserialize a url tree.
+ *
+ * @experimental
  */
 export abstract class UrlSerializer {
   /**
@@ -168,6 +176,8 @@ export abstract class UrlSerializer {
 
 /**
  * A default implementation of the serialization.
+ *
+ * @experimental
  */
 export class DefaultUrlSerializer implements UrlSerializer {
   parse(url: string): UrlTree {
@@ -427,4 +437,4 @@ class UrlParser {
 
     return segments;
   }
-}
+}