Unify manifest.json files

[rugk]

Background

#68 has been solved, but we still have multiple manifest.json files that are a burden to maintain.

Proposed solution

Ease maintenance and unify the files, MDN recommends just specifying all properties in a single manifest.json:

{
  "name": "Demo of service worker + event page",
  "version": "1",
  "manifest_version": 3,
  "background": {
    "scripts": ["background.js"],
    "service_worker": "background.js"
  }
}

Also:

• we can drop the "dev version" manifest then, the benefit was not really big anyway
• Probably use the file chromeServiceWorker.js (renamed to backgroundServiceWorker.js or even just background.js) also for Firefox, so testing is easier and you have less differences between browsers.

Alternatives

Keep as is.

Additional context

Docs: https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/manifest.json/background