publish to GPR as a postpublish step

Author: koddsson

package.json

@@ -15,7 +15,8 @@
     "build": "npm run build-umd && npm run build-esm && cp index.js.flow dist/index.esm.js.flow && cp index.js.flow dist/index.umd.js.flow",
     "pretest": "npm run build",
     "test": "karma start test/karma.config.js",
-    "prepublishOnly": "npm run build"
+    "prepublishOnly": "npm run build",
+    "postpublish": "npm publish --ignore-scripts --@github:registry='https://npm.pkg.github.com'"
   },
   "keywords": [
     "custom-element",