Examine source code of Ctrl+Number to select tabs

{
  "manifest_version": 2,
  "name": "Tab Switch Shortcuts",
  "version": "1.0",
  "description": "Remap Ctrl+number to switch tabs",
  "background": {
    "scripts": [
      "background.js"
    ]
  },
  "commands": {
    "switch-to-tab-1": {
      "suggested_key": {
        "default": "Ctrl+1"
      },
      "description": "Switch to tab 1"
    },
    "switch-to-tab-2": {
      "suggested_key": {
        "default": "Ctrl+2"
      },
      "description": "Switch to tab 2"
    },
    "switch-to-tab-3": {
      "suggested_key": {
        "default": "Ctrl+3"
      },
      "description": "Switch to tab 3"
    },
    "switch-to-tab-4": {
      "suggested_key": {
        "default": "Ctrl+4"
      },
      "description": "Switch to tab 4"
    },
    "switch-to-tab-5": {
      "suggested_key": {
        "default": "Ctrl+5"
      },
      "description": "Switch to tab 5"
    },
    "switch-to-tab-6": {
      "suggested_key": {
        "default": "Ctrl+6"
      },
      "description": "Switch to tab 6"
    },
    "switch-to-tab-7": {
      "suggested_key": {
        "default": "Ctrl+7"
      },
      "description": "Switch to tab 7"
    },
    "switch-to-tab-8": {
      "suggested_key": {
        "default": "Ctrl+8"
      },
      "description": "Switch to tab 8"
    },
    "switch-to-tab-9": {
      "suggested_key": {
        "default": "Ctrl+9"
      },
      "description": "Switch to tab 9"
    }
  },
  "permissions": [
    "tabs"
  ],
  "browser_specific_settings": {
    "gecko": {
      "id": "{fa109781-3edc-49d1-a5bf-bb3c4c1f084f}"
    }
  }
}