feat(challenge20): create challenge testing modal

apps/testing-modal/src/app/error.dialog.ts

@@ -0,0 +1,19 @@
+/* eslint-disable @angular-eslint/component-class-suffix */
+import { Component } from '@angular/core';
+import { MatButtonModule } from '@angular/material/button';
+import { MatDialogModule } from '@angular/material/dialog';
+
+@Component({
+  standalone: true,
+  imports: [MatButtonModule, MatDialogModule],
+  template: `
+    <h1 mat-dialog-title>Error</h1>
+    <div mat-dialog-content>
+      You must enter a <span class="font-bold">name</span> first!!
+    </div>
+    <div mat-dialog-actions>
+      <button mat-button mat-dialog-close>OK</button>
+    </div>
+  `,
+})
+export class ErrorDialog {}