Add Codecov report upload to GitHub workflow

The updated GitHub workflow now includes a step to upload coverage reports to Codecov. This ensures that code coverage metrics are generated and uploaded each time a test run is completed during the CI process. It uses the `codecov/codecov-action@v3` action and the `CODECOV_TOKEN` secret for authentication.

Author: o0h

.github/workflows/phpunit.yml

@@ -16,6 +16,11 @@ jobs:
         run: composer install
       - name: Run phpunit
         run: composer test-unit -- --coverage-clover=coverage.xml
+      - name: Upload coverage reports to Codecov
+        uses: codecov/codecov-action@v3
+        env:
+          CODECOV_TOKEN: ${{ secrets.CODECOV_TOKEN }}
+
   integration-test:
     runs-on: ubuntu-latest
     steps: