Examine source code of Music Play Countdown 123

{
  "description": "Performs a countdown before playing a song.",
  "manifest_version": 2,
  "name": "Music Play Countdown",
  "version": "1.0.1",
  "homepage_url": "https://github.com/vitorog/music-play-countdown",
  "icons": {
    "48": "icons/metronome.svg",
    "96": "icons/metronome.svg"
  },
  "author": "Vitor Gomes",
  "page_action": {
    "default_icon": {
      "19": "icons/metronome-19.png",
      "38": "icons/metronome-38.png"
    },
    "default_title": "Music Play Countdown",
    "default_popup": "src/popup/popup.html",
    "show_matches": [
      "*://www.deezer.com/*",
      "*://open.spotify.com/*"
    ]
  },
  "web_accessible_resources": [
    "sounds/beat.ogg"
  ],
  "content_scripts": [
    {
      "matches": [
        "*://www.deezer.com/*"
      ],
      "js": [
        "src/content-scripts/deezer.js"
      ]
    },
    {
      "matches": [
        "*://open.spotify.com/*"
      ],
      "js": [
        "src/content-scripts/spotify.js"
      ]
    }
  ],
  "browser_specific_settings": {
    "gecko": {
      "id": "{5c178898-d79b-490d-8969-e198e1753257}"
    }
  }
}