Run action on different platforms (#65)

.github/workflows/test.yml

@@ -15,7 +15,13 @@ jobs:
           npm install
           npm run all
   test: # make sure the action works on a clean machine without building
-    runs-on: ubuntu-latest
+    strategy:
+      matrix:
+        os:
+          - ubuntu-latest
+          - macos-latest
+          - windows-latest
+    runs-on: ${{ matrix.os }}
     steps:
       - uses: actions/checkout@v2
       - uses: ./