docs: add example and edit two-way-binding section of Template Syntax (#26278)

PR Close #26278

aio/content/examples/two-way-binding/src/app/app.component.html

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+<h1 id="two-way">Two-way Binding</h1>
+<div id="two-way-1">
+  <!-- #docregion two-way-1 -->
+  <app-sizer [(size)]="fontSizePx"></app-sizer>
+  <div [style.font-size.px]="fontSizePx">Resizable Text</div>
+  <!-- #enddocregion two-way-1 -->
+  <label>FontSize (px): <input [(ngModel)]="fontSizePx"></label>
+</div>
+<br>
+<div id="two-way-2">
+  <h2>De-sugared two-way binding</h2>
+  <!-- #docregion two-way-2 -->
+  <app-sizer [size]="fontSizePx" (sizeChange)="fontSizePx=$event"></app-sizer>
+  <!-- #enddocregion two-way-2 -->
+</div>
+
+

aio/content/examples/two-way-binding/src/app/app.component.spec.ts

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+import { TestBed, async } from '@angular/core/testing';
+import { AppComponent } from './app.component';
+describe('AppComponent', () => {
+  beforeEach(async(() => {
+    TestBed.configureTestingModule({
+      declarations: [
+        AppComponent
+      ],
+    }).compileComponents();
+  }));
+  it('should create the app', async(() => {
+    const fixture = TestBed.createComponent(AppComponent);
+    const app = fixture.debugElement.componentInstance;
+    expect(app).toBeTruthy();
+  }));
+  it(`should have as title 'app'`, async(() => {
+    const fixture = TestBed.createComponent(AppComponent);
+    const app = fixture.debugElement.componentInstance;
+    expect(app.title).toEqual('app');
+  }));
+  it('should render title in a h1 tag', async(() => {
+    const fixture = TestBed.createComponent(AppComponent);
+    fixture.detectChanges();
+    const compiled = fixture.debugElement.nativeElement;
+    expect(compiled.querySelector('h1').textContent).toContain('Welcome to app!');
+  }));
+});

aio/content/examples/two-way-binding/src/app/app.component.ts

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+import { Component } from '@angular/core';
+
+@Component({
+  selector: 'app-root',
+  templateUrl: './app.component.html',
+  styleUrls: ['./app.component.css']
+})
+export class AppComponent {
+  constructor() { }
+  // #docregion font-size
+  fontSizePx = 16;
+  // #enddocregion font-size
+}

aio/content/examples/two-way-binding/src/app/app.module.ts

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+import { BrowserModule } from '@angular/platform-browser';
+import { NgModule } from '@angular/core';
+
+
+import { AppComponent } from './app.component';
+import { SizerComponent } from './sizer/sizer.component';
+import { FormsModule } from '@angular/forms';
+
+
+@NgModule({
+  declarations: [
+    AppComponent,
+    SizerComponent
+  ],
+  imports: [
+    BrowserModule,
+    FormsModule
+  ],
+  providers: [],
+  bootstrap: [AppComponent]
+})
+export class AppModule { }

aio/content/examples/two-way-binding/src/app/sizer/sizer.component.html

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+<div>
+  <button (click)="dec()" title="smaller">-</button>
+  <button (click)="inc()" title="bigger">+</button>
+  <label [style.font-size.px]="size">FontSize: {{size}}px</label>
+</div>

aio/content/examples/two-way-binding/src/app/sizer/sizer.component.spec.ts

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+import { async, ComponentFixture, TestBed } from '@angular/core/testing';
+
+import { SizerComponent } from './sizer.component';
+
+describe('SizerComponent', () => {
+  let component: SizerComponent;
+  let fixture: ComponentFixture<SizerComponent>;
+
+  beforeEach(async(() => {
+    TestBed.configureTestingModule({
+      declarations: [ SizerComponent ]
+    })
+    .compileComponents();
+  }));
+
+  beforeEach(() => {
+    fixture = TestBed.createComponent(SizerComponent);
+    component = fixture.componentInstance;
+    fixture.detectChanges();
+  });
+
+  it('should create', () => {
+    expect(component).toBeTruthy();
+  });
+});

aio/content/examples/two-way-binding/src/app/sizer/sizer.component.ts

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+import { Component, Input, Output, EventEmitter } from '@angular/core';
+
+@Component({
+  selector: 'app-sizer',
+  templateUrl: './sizer.component.html',
+  styleUrls: ['./sizer.component.css']
+})
+export class SizerComponent {
+
+
+  @Input()  size: number | string;
+  @Output() sizeChange = new EventEmitter<number>();
+
+  dec() { this.resize(-1); }
+  inc() { this.resize(+1); }
+
+  resize(delta: number) {
+    this.size = Math.min(40, Math.max(8, +this.size + delta));
+    this.sizeChange.emit(this.size);
+  }
+
+}