chore: move injector to being non-internal but private (#10339)

* chore: move injector to being non-internal but private

* Add the new non-internal method to the public API.

modules/@angular/core/src/linker/compiler.ts

@@ -41,10 +41,8 @@ export class ComponentStillLoadingError extends BaseException {
 export class Compiler {
   /**
    * Returns the injector with which the compiler has been created.
-   *
-   * @internal
    */
-  get injector(): Injector {
+  get _injector(): Injector {
     throw new BaseException(`Runtime compiler is not loaded. Tried to read the injector.`);
   }