Migrate to manifest version 3

[ggrossetie]

https://developer.chrome.com/blog/mv2-transition/

The problem is that currently Firefox does not support background.service_worker: https://developer.chrome.com/docs/extensions/migrating/to-service-workers/. Basically, we cannot use the same manifest nor the same codebase 😞

Another potential issue is that XMLHttpRequest must be replaced by fetch: https://developer.chrome.com/docs/extensions/migrating/to-service-workers/#replace-xmlhttprequest
The problem is that Asciidoctor.js processing is synchronous:

• Include resolution is using synchronous XMLHttpRequest
• Kroki is using synchronous XMLHttpRequest

It might still be possible to use XMLHttpRequest from the page.

[ggrossetie]

It's still possible to upload a new version of an existing extension using manifest v2 on the Chrome WebStore. However, it's not possible on Edge:

[ggrossetie]

• Migration Guide from Firefox: https://extensionworkshop.com/documentation/develop/manifest-v3-migration-guide/
• Migration Guide from Chrome: https://developer.chrome.com/docs/extensions/migrating/