CI: Add prettier step

Author: BenediktSeidl

.github/workflows/ci.yml

@@ -17,5 +17,8 @@ jobs:
       - name: Install dependencies
         run: yarn install --frozen-lockfile
 
+      - name: Run prettier
+        run: yarn prettier -c src test package.json tsconfig.json tsfmt.json jest.config.js .github/workflows/*.yml
+
       - name: Build and test frontend
         run: yarn build