chore: switch to codecov v4

No change to logic. This updates codecov/codecov-action to v4. This
version supposedly has better support for external contributors working
from repository forks.

This also switches to use a CODECOV_TOKEN.

.github/workflows/main.yml

@@ -28,6 +28,8 @@ jobs:
           node-version: ${{ matrix.node-version }}
       - run: npm install
       - run: npm run test
-      - uses: codecov/codecov-action@v2
+      - name: Upload coverage reports to Codecov
+        uses: codecov/codecov-action@v4
         with:
+          token: ${{ secrets.CODECOV_TOKEN }}
           fail_ci_if_error: true