refactor: split app wrapper from focused example code (single-file apps)

Convert 20 more single-file apps to the validated pattern: lib/main.dart
holds only the runApp + MaterialApp bootstrap, while a new lib/example.dart
holds the full screen (Scaffold, AppBar) and the focused example code.

Apps: enabling_splash_screen, getx_counter_app, image_from_network,
infinite_list, load_local_image, load_local_json, persist_key_value,
push_notifications, sliver_app_bar_example, stateless_widgets, tic_tac_toe,
tip_calculator, using_alert_dialog, using_edittext, using_expansionpanel,
using_http_get, using_interactiveviewer, using_snackbar, using_stepper,
using_theme

using_stepper/lib/example.dart

@@ -0,0 +1,94 @@
+import 'package:flutter/material.dart';
+
+// Example: a vertical Stepper with three steps and navigation controls.
+class Example extends StatefulWidget {
+  const Example({super.key});
+
+  @override
+  State<Example> createState() => _ExampleState();
+}
+
+class _ExampleState extends State<Example> {
+  // init the step to 0th position
+  int current_step = 0;
+  List<Step> my_steps = [
+    Step(
+        // Title of the Step
+        title: const Text("Step 1"),
+        // Content, it can be any widget here. Using basic Text for this example
+        content: const Text("Hello!"),
+        isActive: true),
+    Step(
+        title: const Text("Step 2"),
+        content: const Text("World!"),
+        // You can change the style of the step icon i.e number, editing, etc.
+        state: StepState.editing,
+        isActive: true),
+    Step(
+        title: const Text("Step 3"),
+        content: const Text("Hello World!"),
+        isActive: true),
+  ];
+
+  @override
+  Widget build(BuildContext context) {
+    return Scaffold(
+      // Appbar
+      appBar: AppBar(
+        // Title
+        title: const Text("Using Stepper"),
+      ),
+      // Body
+      body: Container(
+          child: Stepper(
+        // Using a variable here for handling the currentStep
+        currentStep: current_step,
+        // List the steps you would like to have
+        steps: my_steps,
+        // Define the type of Stepper style
+        // StepperType.horizontal :  Horizontal Style
+        // StepperType.vertical   :  Vertical Style
+        type: StepperType.vertical,
+        // Know the step that is tapped
+        onStepTapped: (step) {
+          // On hitting step itself, change the state and jump to that step
+          setState(() {
+            // update the variable handling the current step value
+            // jump to the tapped step
+            current_step = step;
+          });
+          // Log function call
+          print("onStepTapped : $step");
+        },
+        onStepCancel: () {
+          // On hitting cancel button, change the state
+          setState(() {
+            // update the variable handling the current step value
+            // going back one step i.e subtracting 1, until its 0
+            if (current_step > 0) {
+              current_step = current_step - 1;
+            } else {
+              current_step = 0;
+            }
+          });
+          // Log function call
+          print("onStepCancel : $current_step");
+        },
+        // On hitting continue button, change the state
+        onStepContinue: () {
+          setState(() {
+            // update the variable handling the current step value
+            // going back one step i.e adding 1, until its the length of the step
+            if (current_step < my_steps.length - 1) {
+              current_step = current_step + 1;
+            } else {
+              current_step = 0;
+            }
+          });
+          // Log function call
+          print("onStepContinue : $current_step");
+        },
+      )),
+    );
+  }
+}