Add action that checks for autosquash commits in pull requests

We want to block merging for these, to ensure that you autosquash
before merging.

Ideally we'd like automatic (or at least via a bot) autosquashing, but
that is not currently possible.

.github/workflows/block-autosquash-commits.yml

@@ -0,0 +1,15 @@
+on: pull_request
+
+name: Pull Requests
+
+jobs:
+  message-check:
+    name: Block Autosquash Commits
+
+    runs-on: ubuntu-latest
+
+    steps:
+      - name: Block Autosquash Commits
+        uses: xt0rted/block-autosquash-commits-action@v2.0.0
+        with:
+          repo-token: ${{ secrets.GITHUB_TOKEN }}