chore: Fail CI build if there are wrongly formatted files (re. #168)

Inspired e.g. by https://github.com/enola-dev/enola/blob/46e6e203d9f8b17406c5b0f5a156ba37c880ab45/tools/git/test.bash#L20.

Author: vorburger

.github/workflows/validation.yml

@@ -39,3 +39,6 @@ jobs:
 
       - name: Compile and test (all) modules with Java ${{ matrix.java-version }}
         run: ./mvnw clean test
+
+      - name: Detected wrongly formatted files
+        run: git status && git diff --exit-code