refactor(language-service): add type guard for NgLanguageService (#40954)

Type guard should be colocated with the `NgLanguageService` interface,
not in `@angular/vscode-ng-language-service`.

PR Close #40954

packages/language-service/api.ts

@@ -43,3 +43,8 @@ export interface NgLanguageService extends ts.LanguageService {
   getTcb(fileName: string, position: number): GetTcbResponse|undefined;
   getComponentLocationsForTemplate(fileName: string): GetComponentLocationsForTemplateResponse;
 }
+
+export function isNgLanguageService(ls: ts.LanguageService|
+                                    NgLanguageService): ls is NgLanguageService {
+  return 'getTcb' in ls;
+}

packages/language-service/ivy/language_service.ts

@@ -247,8 +247,8 @@ export class LanguageService {
     });
   }
 
-  getTcb(fileName: string, position: number): GetTcbResponse {
-    return this.withCompiler<GetTcbResponse>(compiler => {
+  getTcb(fileName: string, position: number): GetTcbResponse|undefined {
+    return this.withCompiler<GetTcbResponse|undefined>(compiler => {
       const templateInfo = getTemplateInfoAtPosition(fileName, position, compiler);
       if (templateInfo === undefined) {
         return undefined;
@@ -496,4 +496,4 @@ function findTightestNodeAtPosition(program: ts.Program, fileName: string, posit
   }
 
   return findTightestNode(sourceFile, position);
-}
+}

packages/language-service/ivy/ts_plugin.ts

@@ -130,7 +130,7 @@ export function create(info: ts.server.PluginCreateInfo): NgLanguageService {
     return diagnostics;
   }
 
-  function getTcb(fileName: string, position: number): GetTcbResponse {
+  function getTcb(fileName: string, position: number): GetTcbResponse|undefined {
     return ngLS.getTcb(fileName, position);
   }