ci: add node 18 to test matrix

Author: no2chem

.github/workflows/tests-sandbox.yml

@@ -5,7 +5,7 @@ jobs:
     strategy:
       matrix:
         platform: [ubuntu-latest, macos-latest]
-        node-version: [14, 16]
+        node-version: [14, 16, 18]
     runs-on: ${{ matrix.platform }}
     steps:
       - uses: actions/checkout@v2