Update .gitignore to new `dart_tool` pub cache

dart-lang/sdk#32030
diff --git a/.gitignore b/.gitignore
index 19c897d..e3348c4 100644
--- a/.gitignore
+++ b/.gitignore
@@ -1,5 +1,6 @@
 out
 packages
+.dart_tool/
 /pubspec.lock
 /test/spec
 /.idea