Added Sample app for SliverAppBar Widget (#117)

* Added Sample app for SliverAppBar Widget

sliver_app_bar_example/lib/main.dart

@@ -0,0 +1,44 @@
+import 'package:flutter/material.dart';
+
+void main() => runApp(const MyApp());
+
+class MyApp extends StatelessWidget {
+  const MyApp({super.key});
+
+  // This widget is the root of your application.
+  @override
+  Widget build(BuildContext context) {
+    return MaterialApp(
+        title: 'Flutter Demo',
+        theme: ThemeData(
+          primarySwatch: Colors.blue,
+        ),
+        home: Scaffold(
+          body: NestedScrollView(
+            floatHeaderSlivers: true,
+            headerSliverBuilder: (context, innerBoxIsScrolled) => [
+              const SliverAppBar(
+                expandedHeight: 240,
+                flexibleSpace: FlexibleSpaceBar(
+                  title: Text('Sliver App Bar Demo'),
+                  background: Image(
+                    image: AssetImage('assets/sample.jpg'),
+                    fit: BoxFit.cover,
+                  ),
+                ),
+                floating: true,
+              ),
+            ],
+            body: ListView.separated(
+                padding: const EdgeInsets.all(12),
+                itemCount: 30,
+                itemBuilder: (context, index) => ListTile(
+                      title: Text('Item $index'),
+                    ),
+                separatorBuilder: (context, index) => const SizedBox(
+                      height: 10,
+                    )),
+          ),
+        ));
+  }
+}