Examine source code of Close Tab on Backspace/Delete

{
  "manifest_version": 2,
  "name": "Close Tab on Backspace/Delete",
  "version": "1.0",
  "description": "Press Backspace or Delete to close the current tab (when not in a text field).",
  "permissions": [
    "tabs"
  ],
  "icons": {
    "16": "icons/icon-16.png",
    "32": "icons/icon-32.png",
    "48": "icons/icon-48.png",
    "96": "icons/icon-96.png"
  },
  "content_scripts": [
    {
      "matches": [
        "<all_urls>"
      ],
      "js": [
        "content.js"
      ],
      "run_at": "document_start"
    }
  ],
  "background": {
    "scripts": [
      "background.js"
    ],
    "persistent": false
  },
  "browser_specific_settings": {
    "gecko": {
      "id": "close-tab-backspace@example.com",
      "strict_min_version": "58.0",
      "data_collection_permissions": {
        "required": [
          "none"
        ],
        "optional": []
      }
    }
  }
}