Add groups to pub ecosystem

Closes #235

GitOrigin-RevId: 5d89afb
Change-Id: I76039984715bcfba961c94cd6d03632fbbfa1b48
Reviewed-on: https://dart-review.googlesource.com/c/dart-docker/+/432184
Reviewed-by: Ivan Inozemtsev <[email protected]>
Commit-Queue: Alexander Thomas <[email protected]>

Author: athomas

.github/dependabot.yml

@@ -20,3 +20,7 @@ updates:
       interval: "monthly"
     labels:
       - type-infra
+    groups:
+      pub:
+        patterns:
+          - "*"