Update template .gitignore rules (#15382)

* Update template .gitignore rules

* Remove old packages rule

* Remove IDE folders from gitignore templates

* Prefer ios.tmpl/.gitignore

packages/flutter_tools/templates/create/.gitignore.tmpl

 .DS_Store
-.atom/
 .dart_tool/
-.idea
-.vscode/
+
 .packages
 .pub/
+
 build/
-ios/.generated/
-packages
+
 .flutter-plugins

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

@@ -12,6 +12,8 @@ build/
 GeneratedPluginRegistrant.h
 GeneratedPluginRegistrant.m
 
+.generated/
+
 *.pbxuser
 *.mode1v3
 *.mode2v3

packages/flutter_tools/templates/package/.gitignore.tmpl

 .DS_Store
-.atom/
 .dart_tool/
-.idea
+
 .packages
 .pub/
-packages
+
+build/
+ios/.generated/
+ios/Flutter/Generated.xcconfig
+ios/Runner/GeneratedPluginRegistrant.*

packages/flutter_tools/templates/plugin/.gitignore.tmpl

 .DS_Store
-.atom/
 .dart_tool/
-.idea
+
 .packages
 .pub/
-build/
-ios/.generated/
-packages
 pubspec.lock
+
+build/

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

@@ -9,6 +9,10 @@ profile
 
 DerivedData/
 build/
+GeneratedPluginRegistrant.h
+GeneratedPluginRegistrant.m
+
+.generated/
 
 *.pbxuser
 *.mode1v3
@@ -29,3 +33,4 @@ xcuserdata
 Icon?
 .tags*
 
+/Flutter/Generated.xcconfig