Examine source code of Unpromote Twitter

{
  "manifest_version": 2,
  "name": "Unpromote Twitter",
  "version": "1.2",
  "description": "Removes promoted tweets from the Twitter timeline.",
  "icons": {
    "48": "icons/icon48.png"
  },
  "permissions": [
    "*://twitter.com/*"
  ],
  "background": {
    "scripts": [
      "unsponsorTwitterBackground.js"
    ]
  }
}