Examine source code of Gutencount

{
  "manifest_version": 2,
  "name": "Gutencount universal word counter",
  "short_name": "Gutencount",
  "version": "1.6.7",
  "developer": {
    "name": "quartertone",
    "url": "https://vox.quartertone.net/"
  },
  "description": "Perform accurate an accurate word count on any webpage or selection, with special focus on Gutenberg ebooks.",
  "icons": {
    "48": "img/gutencount-logo.svg",
    "96": "img/gutencount-logo.svg"
  },
  "browser_specific_settings": {
    "gecko": {
      "id": "gutencount@vox.quartertone.net"
    }
  },
  "content_scripts": [
    {
      "matches": [
        "*://*.gutenberg.org/*"
      ],
      "js": [
        "gutencount.js",
        "action.js"
      ]
    }
  ]
}