Add automatic check for not using nullptr etc in 3.2 branch

This is similar to the check in master but here it checks that the C++
keywords are not used, as we still support C++98 in this branch.

.github/workflows/code_checks.yml

@@ -74,3 +74,22 @@ jobs:
       - name: Check for mixed EOL
         run: |
             ./misc/scripts/check_mixed_eol.sh
+
+  check-cxx-style:
+    runs-on: ubuntu-20.04
+    name: Check C++ Style
+
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v3
+
+      - name: Check for C++11 keywords
+        run: |
+            git fetch --depth=1 origin master
+            if git diff origin/master \
+               ':!.github/workflows/code_checks.yml' \
+               ':!src/stc/scintilla/' \
+               | grep -E '(override|noexcept|nullptr[^_])'; then
+              echo "::error ::Please use C++98 equivalents of the C++11 keywords in this branch."
+              exit 1
+            fi