Add CI test that fails if a PR is issued to main (#1356)

The `edited` activity type includes changes to the base branch, so we
need to add that to the trigger types. It also needs to run on
development as well as main, so the workflow can succeed once the base
branch is changed.

Do the check in bash, so we get "Successful" instead of "Skipped"
Co-authored-by: Eric T. Johnson <yut23@users.noreply.github.com>

.github/workflows/check_pr_branch.yml

@@ -0,0 +1,20 @@
+name: check PR branch
+
+on:
+  pull_request:
+    types:
+      - opened
+      - synchronize
+      - reopened
+      - edited
+
+jobs:
+  check-PR-branch:
+    runs-on: ubuntu-latest
+    steps:
+      - name: PRs should not target main
+        run: |
+          if [[ "${{ github.base_ref }}" == "main" ]]; then
+            echo 'Pull requests must not be made against main. Please target development instead.'
+            exit 1
+          fi