Implement POC: web ext background page that can request article HTML

[th0rgall]

Constraint: use manifest v3, the future of web exts.

The POC should (necessary goals):

1. Be able to claim wide permissions to access all websites without being blocked by CORS rules.
2. Be able to request article HTML of any website.

Ideally, it should also (stretch goal):

1. Be able to modify and inject HTTP headers that are generally not accessible in browser fetch requests, like referer, using the new declarativeNetRequest API. This used to be possible.
2. Be able to perform the request on behalf of a hypothetical https://readup.org web page, using web-page <-> extensions message passing, displaying (parsed) article HTML in te requesting web page. (@th0rgall implemented a similar POC in manifest v2 once).
3. Be able to perform the requests with or without the browser's user credentials/cookies.

The impact would be significant:

• With the necessary goals + stretch goal 1 & 2, the web ext could reach feature parity compared to the desktop apps. This would enable us to:
  • Deprecate all desktop apps & related maintenance costs
  • Make onboarding easier
  • Make Readup web-native again (as many hardcore users have wanted!)
• With stretch goal 3, it could surpass the current desktop apps by being able to request paywalled articles from sites that people pay for..

[th0rgall]

Done! https://github.com/reallyreadit/web-ext-poc -> it meets all the goals.