run on windows, mac, and linux

Author: catdad

.github/workflows/ci.yml

@@ -15,7 +15,7 @@ jobs:
     strategy:
       matrix:
         node-version: [8, 10, 12, 14]
-        os: [ubuntu-latest]
+        os: [ubuntu-latest, windows-latest, macOS-latest]
     steps:
       - uses: actions/checkout@v2
       - name: Use Node.js ${{ matrix.node-version }}