CI: add codecov upload

Author: MikeSpa

.github/workflows/ci.yml

@@ -28,7 +28,12 @@ jobs:
         run: npm run typecheck
 
       - name: Test
-        run: npm test
+        run: npm test:coverage
+
+      - name: Upload coverage reports to Codecov
+        uses: codecov/codecov-action@v5
+        with:
+          token: ${{ secrets.CODECOV_TOKEN }}
 
       - name: Build
         run: npm run build