Update .gitignore to new `dart_tool` pub cache

See: https://github.com/dart-lang/sdk/issues/32030
diff --git a/.gitignore b/.gitignore
index a8b93ef..4b1d878 100644
--- a/.gitignore
+++ b/.gitignore
@@ -1,5 +1,5 @@
 .packages
-.pub
+.dart_tool
 packages
 pubspec.lock
 doc/api/