ci: add dependabot (pubgrub-rs#196)

This is the same configuration we are using in uv minus the automatic lable: weekly updates for dependabot and cargo dependencies. Feel to edit the interval if you prefer something else.

Author: konstin

.github/dependabot.yml

@@ -0,0 +1,16 @@
+version: 2
+updates:
+  - package-ecosystem: "github-actions"
+    directory: "/"
+    schedule:
+      interval: "weekly"
+    groups:
+      artifacts:
+        # Group upload/download artifact updates, the versions are dependent
+        patterns:
+          - "actions/*-artifact"
+
+  - package-ecosystem: "cargo"
+    directory: "/"
+    schedule:
+      interval: "weekly"