Manifest v3

[frigus02]

The following features are probably not possible with manifest v3:

• Show original (unprocessed by browser) response headers, status code and status text.
• Remove default request header content-type
  • TODO: can we remove content-type if the body is not formData?
• TODO: test merging of browser cookie and custom cookie header
• TODO: with v3 Firefox seems to upgrade all requests to http because of the changed Content Security Policy
• TODO: background pages (or service workers) are not persistent anymore. This means the port in rester.js can and will disconnect after a while of inactivity. How do we deal with this? Ideally I'd want to reconnect only when needed. But at that time we may have missed a few events (settingsChanged or dataChanged), so the page is out of date. We need to bring it up to date again.

Today (2023-03-14) Chrome and Firefox differ in the APIs they support for Manifest v3:

• Chrome requires background.serice_worker. Firefox requires background.scripts. Luckily RESTer doesn't need any serice worker specific API. The same script runs fine with either API.
• Chrome dropped support for webRequest and requires declarativeNetRequest. Firefox does not support declarativeNetRequest, yet, but still supports webRequest.

Fixes #874