ci: add Codecov

Author: cleot

.github/workflows/build.yml

@@ -33,3 +33,8 @@ jobs:
 
       - name: Build
         run: npm run build
+
+      - name: Upload coverage to Codecov
+        uses: codecov/codecov-action@v5
+        with:
+          token: ${{ secrets.CODECOV_TOKEN }}