ci: add node 18 (#5)

Author: juliangruber

.github/workflows/ci.yml

@@ -5,7 +5,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        node: ['14', '16']
+        node: ['14', '16', '18']
     steps:
       - uses: actions/checkout@v2
       - uses: actions/setup-node@v2