feat(language-service): add definitions for templateUrl (#32238)

Adds support for `getDefinitionAt` when called on a templateUrl
property assignment.

The currrent architecture for getting definitions is designed to be
called on templates, so we have to introduce a new
`getTsDefinitionAndBoundSpan` method to get Angular-specific definitions
in TypeScript files and pass a `readTemplate` closure that will read the
contents of a template using `TypeScriptServiceHost#getTemplates`. We
can probably go in and make this nicer in a future PR, though I'm not
sure what the best architecture should be yet.

Part of angular/vscode-ng-language-service#111

PR Close #32238

packages/language-service/src/language_service.ts

@@ -10,7 +10,7 @@ import * as tss from 'typescript/lib/tsserverlibrary';
 
 import {isAstResult} from './common';
 import {getTemplateCompletions, ngCompletionToTsCompletionEntry} from './completions';
-import {getDefinitionAndBoundSpan} from './definitions';
+import {getDefinitionAndBoundSpan, getTsDefinitionAndBoundSpan} from './definitions';
 import {getDeclarationDiagnostics, getTemplateDiagnostics, ngDiagnosticToTsDiagnostic, uniqueBySpan} from './diagnostics';
 import {getHover} from './hover';
 import {Diagnostic, LanguageService} from './types';
@@ -80,6 +80,15 @@ class LanguageServiceImpl implements LanguageService {
     if (templateInfo) {
       return getDefinitionAndBoundSpan(templateInfo, position);
     }
+
+    // Attempt to get Angular-specific definitions in a TypeScript file, like templates defined
+    // in a `templateUrl` property.
+    if (fileName.endsWith('.ts')) {
+      const sf = this.host.getSourceFile(fileName);
+      if (sf) {
+        return getTsDefinitionAndBoundSpan(sf, position, this.host.host);
+      }
+    }
   }
 
   getHoverAt(fileName: string, position: number): tss.QuickInfo|undefined {