Add rebase check workflow.

Author: ktlim

.github/workflows/rebase_checker.yaml

@@ -0,0 +1,8 @@
+---
+name: Check that 'main' is not merged into the development branch
+
+on: pull_request
+
+jobs:
+  call-workflow:
+    uses: lsst/rubin_workflows/.github/workflows/rebase_checker.yaml@main