feat(language-service): provide diagnostic for invalid templateUrls (#32586)

`templateUrls` that do not point to actual files are now diagnosed as such
by the Language Service. Support for `styleUrls` will come in a next PR.

This introduces a utility method `getPropertyValueOfType` that scans
TypeScript ASTs until a property assignment whose initializer of a
certain type is found. This PR also notices a couple of things that
could be improved in the language-service implementation, such as
enumerating directive properties and unifying common logic, that will be
fixed in future PRs.

Part of #32564.

PR Close #32586

packages/language-service/test/diagnostics_spec.ts

@@ -475,6 +475,40 @@ describe('diagnostics', () => {
             `Module '"../node_modules/@angular/core/core"' has no exported member 'OpaqueToken'.`);
   });
 
+  describe('URL diagnostics', () => {
+    it('should report errors for invalid templateUrls', () => {
+      const fileName = mockHost.addCode(`
+	@Component({
+          templateUrl: '«notAFile»',
+        })
+        export class MyComponent {}`);
+
+      const marker = mockHost.getReferenceMarkerFor(fileName, 'notAFile');
+
+      const diagnostics = ngLS.getDiagnostics(fileName) !;
+      const urlDiagnostic =
+          diagnostics.find(d => d.messageText === 'URL does not point to a valid file');
+      expect(urlDiagnostic).toBeDefined();
+
+      const {start, length} = urlDiagnostic !;
+      expect(start).toBe(marker.start);
+      expect(length).toBe(marker.length);
+    });
+
+    it('should not report errors for valid templateUrls', () => {
+      const fileName = mockHost.addCode(`
+	@Component({
+          templateUrl: './test.ng',
+	})
+	export class MyComponent {}`);
+
+      const diagnostics = ngLS.getDiagnostics(fileName) !;
+      const urlDiagnostic =
+          diagnostics.find(d => d.messageText === 'URL does not point to a valid file');
+      expect(urlDiagnostic).toBeUndefined();
+    });
+  });
+
   // https://github.com/angular/vscode-ng-language-service/issues/235
   // There is no easy fix for this issue currently due to the way template
   // tokenization is done. In the example below, the whole string