Add contrib/checkformat.bash to check for clang-formatting (#1646)

.github/workflows/checkdiff.yml

@@ -11,7 +11,7 @@ jobs:
           cd rgbds
           git remote add upstream "${{ github.event.pull_request.base.repo.clone_url }}"
           git fetch upstream
-      - name: Checkdiff
+      - name: Check diff
         working-directory: rgbds
         run: |
           make checkdiff "BASE_REF=${{ github.event.pull_request.base.sha }}" Q= | tee log

.github/workflows/checkformat.yml

@@ -0,0 +1,12 @@
+name: Code format checking
+on: pull_request
+
+jobs:
+  checkformat:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout repo
+        uses: actions/checkout@v4
+      - name: Check format
+        run: |
+          ./contrib/checkformat.bash