ci: add node 15

Author: cossssmin

.github/workflows/nodejs.yml

@@ -15,7 +15,7 @@ jobs:
 
     strategy:
       matrix:
-        node-version: [10, 12, 14]
+        node-version: [10, 12, 14, 15]
 
     steps:
       - uses: actions/checkout@v2