Examine source code of QuickDef: Instant Word Lookup

{
  "manifest_version": 3,
  "name": "Word Definer",
  "version": "1.3",
  "description": "Instant dictionary: double-click any word on web pages to see its definition. Persistent sidebar for local files and PDFs.",
  "permissions": [
    "storage",
    "activeTab",
    "scripting",
    "contextMenus"
  ],
  "host_permissions": [
    "https://api.datamuse.com/*",
    "<all_urls>"
  ],
  "action": {
    "default_title": "Word Definer",
    "default_popup": "popup.html"
  },
  "sidebar_action": {
    "default_title": "Word Definer",
    "default_panel": "popup.html"
  },
  "background": {
    "scripts": [
      "background.js"
    ]
  },
  "content_scripts": [
    {
      "matches": [
        "http://*/*",
        "https://*/*",
        "file:///*"
      ],
      "js": [
        "content.js"
      ],
      "css": [
        "content.css"
      ],
      "run_at": "document_idle"
    }
  ],
  "browser_specific_settings": {
    "gecko": {
      "id": "word-definer@praneet.local"
    }
  }
}