Popping the last page of the flow should complete the flow

**Is your feature request related to a problem? Please describe.**
Since using Navigator.pop() goes back in the flow, it would be nice if this remains consistent even when the flow only has one page. I would expect popping the last page to complete the flow.

Having the above functionality makes nesting FlowBuilders very convenient.

Example

```dart
import 'package:flow_builder/flow_builder.dart';
import 'package:flutter/material.dart';

void main() {
runApp(const MaterialApp(home: _HomeFlow()));
}

enum _HomeFlowState {
home,
settings,
}

class _HomeFlow extends StatelessWidget {
const _HomeFlow({Key? key}) : super(key: key);

static List _onGeneratePages(_HomeFlowState state, List pages) {
switch (state) {
case _HomeFlowState.home:
return [_HomePage.page()];
case _HomeFlowState.settings:
return [_HomePage.page(), _SettingsFlow.page()];
}
}

@override
Widget build(BuildContext context) {
return const FlowBuilder(
state: _HomeFlowState.home,
onGeneratePages: _onGeneratePages,
);
}
}

class _HomePage extends StatelessWidget {
const _HomePage({Key? key}) : super(key: key);

static Page page() => const MaterialPage(child: _HomePage());

@override
Widget build(BuildContext context) {
return Scaffold(
appBar: AppBar(
actions: [
IconButton(
onPressed: () => context
.flow<_HomeFlowState>()
.update((_) => _HomeFlowState.settings),
icon: const Icon(Icons.settings),
),
],
),
body: const Center(child: Text('home')),
);
}
}

enum _SettingsFlowState {
settings,
about,
support,
}

class _SettingsFlow extends StatelessWidget {
const _SettingsFlow({Key? key}) : super(key: key);

static Page page() => const MaterialPage(child: _SettingsFlow());

static List _onGeneratePages(
_SettingsFlowState state, List pages) {
switch (state) {
case _SettingsFlowState.settings:
return [_SettingsPage.page()];
case _SettingsFlowState.about:
return [_SettingsPage.page(), _AboutPage.page()];
case _SettingsFlowState.support:
return [_SettingsPage.page(), _SupportPage.page()];
}
}

@override
Widget build(BuildContext context) {
return const FlowBuilder(
state: _SettingsFlowState.settings,
onGeneratePages: _onGeneratePages,
);
}
}

class _SettingsPage extends StatelessWidget {
const _SettingsPage({Key? key}) : super(key: key);

static Page page() => const MaterialPage(child: _SettingsPage());

@override
Widget build(BuildContext context) {
// Native pop should also complete the flow.
return Scaffold(
appBar: AppBar(
leading: BackButton(onPressed: () => Navigator.of(context).pop()), // Should complete the flow
title: const Text('Settings'),
),
body: Center(
child: Column(
children: [
TextButton(
onPressed: () => context
.flow<_SettingsFlowState>()
.update((_) => _SettingsFlowState.about),
child: const Text('About'),
),
TextButton(
onPressed: () => context
.flow<_SettingsFlowState>()
.update((_) => _SettingsFlowState.support),
child: const Text('Support'),
),
],
),
),
);
}
}

class _AboutPage extends StatelessWidget {
const _AboutPage({Key? key}) : super(key: key);

static Page page() => const MaterialPage(child: _AboutPage());

@override
Widget build(BuildContext context) {
return Scaffold(
appBar: AppBar(title: const Text('About')),
body: const Center(child: Text('About')),
);
}
}

class _SupportPage extends StatelessWidget {
const _SupportPage({Key? key}) : super(key: key);

static Page page() => const MaterialPage(child: _SupportPage());

@override
Widget build(BuildContext context) {
return Scaffold(
appBar: AppBar(title: const Text('Support')),
body: const Center(child: Text('Support')),
);
}
}
```

**Describe the solution you'd like**
I would like to have some logic that makes the flow complete whenever the last page of the flow is popped.

**Describe alternatives you've considered**
N/A

**Additional context**
N/A