Move iOS and Android gitignore rules into folders (#40864)

Rather than have a monolithic .gitignore covering all platforms in the
app template, have each platform folder contain its own rules.

Fixes #40793

packages/flutter_tools/templates/app/.gitignore.tmpl

@@ -29,48 +29,8 @@
 .pub/
 /build/
 
-# Android related
-**/android/**/gradle-wrapper.jar
-**/android/.gradle
-**/android/captures/
-**/android/gradlew
-**/android/gradlew.bat
-**/android/local.properties
-**/android/**/GeneratedPluginRegistrant.java
-
-# iOS/XCode related
-**/ios/**/*.mode1v3
-**/ios/**/*.mode2v3
-**/ios/**/*.moved-aside
-**/ios/**/*.pbxuser
-**/ios/**/*.perspectivev3
-**/ios/**/*sync/
-**/ios/**/.sconsign.dblite
-**/ios/**/.tags*
-**/ios/**/.vagrant/
-**/ios/**/DerivedData/
-**/ios/**/Icon?
-**/ios/**/Pods/
-**/ios/**/.symlinks/
-**/ios/**/profile
-**/ios/**/xcuserdata
-**/ios/.generated/
-**/ios/Flutter/App.framework
-**/ios/Flutter/Flutter.framework
-**/ios/Flutter/Generated.xcconfig
-**/ios/Flutter/app.flx
-**/ios/Flutter/app.zip
-**/ios/Flutter/flutter_assets/
-**/ios/Flutter/flutter_export_environment.sh
-**/ios/ServiceDefinitions.json
-**/ios/Runner/GeneratedPluginRegistrant.*
-
 # Web related
 lib/generated_plugin_registrant.dart
 
 # Exceptions to above rules.
-!**/ios/**/default.mode1v3
-!**/ios/**/default.mode2v3
-!**/ios/**/default.pbxuser
-!**/ios/**/default.perspectivev3
 !/packages/flutter_tools/test/data/dart_dependencies_test/**/.packages

packages/flutter_tools/templates/app/android.tmpl/.gitignore

+gradle-wrapper.jar
+/.gradle
+/captures/
+/gradlew
+/gradlew.bat
+/local.properties
+GeneratedPluginRegistrant.java

packages/flutter_tools/templates/app/ios.tmpl/.gitignore

+*.mode1v3
+*.mode2v3
+*.moved-aside
+*.pbxuser
+*.perspectivev3
+**/*sync/
+.sconsign.dblite
+.tags*
+**/.vagrant/
+**/DerivedData/
+Icon?
+**/Pods/
+**/.symlinks/
+profile
+xcuserdata
+**/.generated/
+Flutter/App.framework
+Flutter/Flutter.framework
+Flutter/Generated.xcconfig
+Flutter/app.flx
+Flutter/app.zip
+Flutter/flutter_assets/
+Flutter/flutter_export_environment.sh
+ServiceDefinitions.json
+Runner/GeneratedPluginRegistrant.*
+
+# Exceptions to above rules.
+!default.mode1v3
+!default.mode2v3
+!default.pbxuser
+!default.perspectivev3