Examine source code of Steam18DB

{
  "manifest_version": 3,
  "name": "Steam18DB",
  "version": "0.1",
  "host_permissions": [
    "https://steam18db.com/*"
  ],
  "background": {
    "service_worker": "scripts/background.js",
    "scripts": [
      "scripts/background.js"
    ]
  },
  "permissions": [
    "storage"
  ],
  "content_scripts": [
    {
      "run_at": "document_end",
      "matches": [
        "*://*.store.steampowered.com/app/*"
      ],
      "js": [
        "scripts/app.js"
      ]
    }
  ],
  "browser_specific_settings": {
    "gecko": {
      "id": "fcolecumberri.steam18db_extension@github.com",
      "strict_min_version": "109.0"
    }
  }
}