Add GitHub action to disallow direct PRs to main (ESCOMP#285)

Originator(s): jimmielin

Description (include issue title and the keyword ['closes', 'fixes',
'resolves'] and issue number):
- adds a GitHub action to disallow branches other than `development` to
have a PR to `main`

List all namelist files that were added or changed: N/A

List all files eliminated and why: N/A

List all files added and what they do:
```
.github/workflows/no-direct-pr-to-main.yaml
  - add GitHub workflow
```

List all existing files that have been modified, and describe the
changes:
(Helpful git command: `git diff --name-status
development...<your_branch_name>`)
N/A

List all automated tests that failed, as well as an explanation for why
they weren't fixed:
N/A

Is this an answer-changing PR? If so, is it a new physics package,
algorithm change, tuning change, etc? N/A - GitHub actions only, no code

If yes to the above question, describe how this code was validated with
the new/modified features:

Author: hplin-ucar

.github/workflows/no-direct-pr-to-main.yaml

@@ -0,0 +1,20 @@
+name: PRs to main must be from development branch
+on:
+  pull_request_target:
+    types:
+      - opened
+      - reopened
+      - synchronize
+      - edited
+    branches:
+      - main
+jobs:
+  check-branches:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Check branches
+        run: |
+          if [ ${{ github.head_ref }} != "development" ]; then
+            echo "Merge requests to main branch are only allowed from the development branch."
+            exit 1
+          fi