message: Add `watch:firefox` npm script
diff: "test:js": "ava",
  "build": "webpack --mode=production",
  "watch": "webpack --mode=development --watch",
  + "watch:firefox": "web-ext run --source-dir=distribution",
  "release:amo": "cd distribution && web-ext-submit",
  "release:cws": "cd distribution && webstore upload --auto-publish",
  "release": "run-s build update-version save-url release:*",
  "ts-node": "^8.1.0",
  "typescript": "^3.4.4",
  "utc-version": "^1.1.0",
  + "web-ext": "^3.0.0",
  "web-ext-submit": "^3.0.0",
  "webpack": "^4.30.0",
  "webpack-cli": "^3.3.1",
  