Examine source code of Cache Longer

{
  "manifest_version": 2,
  "name": "Cache Longer",
  "version": "1.0",
  "author": "Rimu Atkinson",
  "icons": {
    "96": "icon.png"
  },
  "description": "Cache Longer modifies HTTP headers to make your browser cache things for longer.",
  "homepage_url": "https://rimu.geek.nz",
  "background": {
    "scripts": [
      "cache_longer.js"
    ]
  },
  "permissions": [
    "storage",
    "webRequest",
    "webRequestBlocking",
    "<all_urls>"
  ]
}