Examine source code of Decode base64 Extension

{
  "manifest_version": 3,
  "name": "Decode base64 Extension",
  "version": "1.0",
  "description": "Decodes selected text using Base64 or URL decoding and replaces it with the original and decoded text (with a link if applicable). Right Click on your selected text and choose 'Decode String'.",
  "permissions": [
    "contextMenus",
    "scripting",
    "activeTab"
  ],
  "background": {
    "scripts": [
      "background.js"
    ]
  },
  "browser_specific_settings": {
    "gecko": {
      "id": "underlines@gmail.com"
    }
  },
  "icons": {
    "512": "icons/icon-512.png"
  }
}