Added icons library and icons example page code generator(python)

1. Automatically generate platform icons extension class (library file) with the same name between Material and Cupertino to `lib/src/platform_icons_gen.dart` file with sorted by name by dart's `Extension`, without changing the existing code(`lib/src/platform_icons.dart`)

2. Automatically generate example page of all icons to file `example/lib//icons_page.dart` with sorted by name, from file `lib/src/platform_icons.dart` and `lib/src/platform_icons_gen.dart`.

Author: bookshiyi

.gitignore

@@ -12,4 +12,6 @@ ios/Runner/GeneratedPluginRegistrant.*
 ios/Flutter/flutter_export_environment.sh
 
 pubspec.lock
-example/ios/Flutter/flutter_export_environment.sh
+example/ios/Flutter/flutter_export_environment.sh
+
+__pycache__

example/lib/icons_page.dart

@@ -30,6 +30,7 @@ export 'src/platform_dialog_action.dart';
 export 'src/platform_elevated_button.dart';
 export 'src/platform_icon_button.dart';
 export 'src/platform_icons.dart';
+export 'src/platform_icons_gen.dart';
 export 'src/platform_nav_bar.dart';
 export 'src/platform_page.dart';
 export 'src/platform_page_route.dart';

lib/flutter_platform_widgets.dart

@@ -188,20 +188,24 @@ class PlatformIcons {
       isMaterial(context) ? Icons.cloud : CupertinoIcons.cloud_fill;
 
   /// Icons: Icons.cloud_download_outlined : CupertinoIcons.cloud_download
-  IconData get cloudDownload =>
-      isMaterial(context) ? Icons.cloud_download_outlined : CupertinoIcons.cloud_download;
+  IconData get cloudDownload => isMaterial(context)
+      ? Icons.cloud_download_outlined
+      : CupertinoIcons.cloud_download;
 
   /// Icons: Icons.cloud_download : CupertinoIcons.cloud_download_fill
-  IconData get cloudDownloadSolid =>
-      isMaterial(context) ? Icons.cloud_download : CupertinoIcons.cloud_download_fill;
+  IconData get cloudDownloadSolid => isMaterial(context)
+      ? Icons.cloud_download
+      : CupertinoIcons.cloud_download_fill;
 
   /// Icons: Icons.cloud_upload_outlined : CupertinoIcons.cloud_upload
-  IconData get cloudUpload =>
-      isMaterial(context) ? Icons.cloud_upload_outlined : CupertinoIcons.cloud_upload;
+  IconData get cloudUpload => isMaterial(context)
+      ? Icons.cloud_upload_outlined
+      : CupertinoIcons.cloud_upload;
 
   /// Icons: Icons.cloud_upload : CupertinoIcons.cloud_upload_fill
-  IconData get cloudUploadSolid =>
-      isMaterial(context) ? Icons.cloud_upload : CupertinoIcons.cloud_upload_fill;
+  IconData get cloudUploadSolid => isMaterial(context)
+      ? Icons.cloud_upload
+      : CupertinoIcons.cloud_upload_fill;
 
   /// Icons: Icons.collections : CupertinoIcons.collections
   IconData get collections =>