ci: add test with node runtime 18 and 20 (#259)

GH action is only running on node 14 and 16 - which are both fully outdated and end-of-life.

.github/workflows/node.js.yml

@@ -9,7 +9,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        node-version: [14.x, 16.x]
+        node-version: [14.x, 16.x, 18.x, 20.x]
     steps:
     - uses: actions/checkout@v2
     - name: Use Node.js ${{ matrix.node-version }}