ci: update to test Node 16.x (#90)

Author: owenvoke

.github/workflows/test.yml

@@ -13,7 +13,7 @@ jobs:
     strategy:
       matrix:
         os: [ubuntu-latest, macos-latest] # todo: windows-latest
-        node-version: [10.x, 12.x, 14.x, 15.x]
+        node-version: [10.x, 12.x, 14.x, 15.x, 16.x]
 
     name: Node ${{ matrix.node-version }} - ${{ matrix.os }}