Examine source code of Age Calculator

{
  "manifest_version": 3,
  "name": "Age Calculator",
  "version": "1.0",
  "description": "Calculate age in years, months, and days",
  "action": {
    "default_popup": "popup.html",
    "default_icon": {
      "16": "icon16.png",
      "48": "icon48.png",
      "128": "icon128.png"
    }
  },
  "icons": {
    "16": "icon16.png",
    "48": "icon48.png",
    "128": "icon128.png"
  },
  "browser_specific_settings": {
    "gecko": {
      "id": "age-converter@unique.extension.id"
    }
  }
}