build(deps-dev): Bump webpack from 4.38.0 to 4.39.0 in /addon

Bumps [webpack](https://github.com/webpack/webpack) from 4.38.0 to 4.39.0.
- [Release notes](https://github.com/webpack/webpack/releases)
- [Commits](webpack/webpack@v4.38.0...v4.39.0)

Signed-off-by: dependabot-preview[bot] <[email protected]>

Author: dependabot-preview[bot]

addon/package-lock.json

@@ -12,7 +12,7 @@
     "eslint-plugin-mozilla": "^0.9.0",
     "eslint-plugin-no-unsanitized": "^3.0.2",
     "web-ext": "^3.1.1",
-    "webpack": "^4.38.0",
+    "webpack": "^4.39.0",
     "webpack-cli": "^3.3.6"
   },
   "scripts": {