Examine source code of MetaMask Legacy Web3

{
  "name": "MetaMask Legacy Web3",
  "short_name": "Legacy Web3",
  "version": "1.0.2",
  "description": "Adds MetaMask's legacy window.web3 API to your browser",
  "author": "https://metamask.io",
  "content_scripts": [
    {
      "matches": [
        "file://*/*",
        "http://*/*",
        "https://*/*"
      ],
      "js": [
        "contentscript.js"
      ],
      "run_at": "document_start",
      "all_frames": true
    }
  ],
  "icons": {
    "16": "images/icon-16.png",
    "48": "images/icon-48.png",
    "128": "images/icon-128.png"
  },
  "manifest_version": 2,
  "browser_specific_settings": {
    "gecko": {
      "id": "legacyweb3@metamask.io",
      "strict_min_version": "56.0"
    }
  }
}