test: Add Node 20 to CI matrix (#160)

.github/workflows/ci.yml

@@ -9,6 +9,10 @@ on:
 jobs:
   tests:
     runs-on: ubuntu-latest
+    strategy:
+      matrix:
+        node: [18, 20]
+    continue-on-error: ${{ matrix.node == 20 }}
     steps:
     - name: Checkout
       uses: actions/checkout@v4
@@ -19,7 +23,7 @@ jobs:
     - name: Setup Nodejs
       uses: actions/setup-node@v4
       with:
-        node-version: ${{ env.NODE_VER }}
+        node-version: ${{ matrix.node }}
     - name: Install dependencies
       run: npm ci
     - name: Lint