refactor(language-service): initial use of type-only imports and standard server types

This commit converts the typescript import in api.ts to a type-only import
and updates the plugin factory to utilize standard TypeScript server types.
Internal interfaces and specific service types are replaced with general
types to decouple the factory wrapper from internal implementations.

Author: clydin

packages/language-service/api.ts

@@ -12,7 +12,7 @@
  * Entry point for all public APIs of the language service package.
  */
 
-import ts from 'typescript';
+import type ts from 'typescript';
 
 export interface PluginConfig {
   /**

packages/language-service/plugin-factory.ts

@@ -9,25 +9,19 @@
 // Note: use a type-only import to prevent TypeScript from being bundled in.
 import type ts from 'typescript';
 
-import {NgLanguageService, PluginConfig} from './api';
-
-interface PluginModule extends ts.server.PluginModule {
-  create(createInfo: ts.server.PluginCreateInfo): NgLanguageService;
-  onConfigurationChanged?(config: PluginConfig): void;
-}
-
-export const factory: ts.server.PluginModuleFactory = (tsModule): PluginModule => {
-  let plugin: PluginModule;
+export const factory: ts.server.PluginModuleFactory = (tsModule) => {
+  let plugin: ts.server.PluginModule;
 
   return {
-    create(info: ts.server.PluginCreateInfo): NgLanguageService {
+    create(info: ts.server.PluginCreateInfo): ts.LanguageService {
       plugin ??= require(`@angular/language-service/bundles/language-service.js`)(tsModule);
+
       return plugin.create(info);
     },
     getExternalFiles(project: ts.server.Project): string[] {
       return plugin?.getExternalFiles?.(project, tsModule.typescript.ProgramUpdateLevel.Full) ?? [];
     },
-    onConfigurationChanged(config: PluginConfig): void {
+    onConfigurationChanged(config: unknown): void {
       plugin?.onConfigurationChanged?.(config);
     },
   };