Use NPM prepublishOnly script instead of prepublish

* Prepublish script runs on local npm installations as well. In our case this script checks the validity of python code. But this code is most likely not to exist at that time. PrepublishOnly event is used on `npm publish` only.

Author: Vadim Frolov

{{cookiecutter.project_shortname}}/package.json

@@ -16,7 +16,7 @@
   "scripts": {
     "start": "webpack-serve --config ./webpack.serve.config.js --open",
     "validate-init": "python _validate_init.py",
-    "prepublish": "npm run validate-init",
+    "prepublishOnly": "npm run validate-init",
     "build:js": "webpack --mode production",
     "build:py_and_r": "dash-generate-components ./src/lib/components {{ cookiecutter.project_shortname }} -p package-info.json --r-prefix '{{ cookiecutter.r_prefix }}'",
     "build:py_and_r-activated": "(. venv/bin/activate || venv\\scripts\\activate && npm run build:py_and_r)",