Add .dart_tool to gitignore (#14932)

Dart is migrating to .dart_tool/ as the location for cached artifacts
and other temporary files generated by tooling in the SDK. As part of
this, pub will be migrating from .pub to .dart_tool/pub.

In future, this path will also be used for other tooling, such as package:build.

.gitignore

@@ -33,6 +33,7 @@ version
 
 # Flutter/Dart/Pub related
 **/doc/api/
+.dart_tool/
 .flutter-plugins
 .packages
 .pub-cache/

dev/tools/vitool/.gitignore

 # Files and directories created by pub
+.dart_tool/
 .packages
 .pub/
 build/

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

 .DS_Store
 .atom/
+.dart_tool/
 .idea
 .vscode/
 .packages

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

 .DS_Store
 .atom/
+.dart_tool/
 .idea
 .packages
 .pub/

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

 .DS_Store
 .atom/
+.dart_tool/
 .idea
 .packages
 .pub/