Allow extensions to register full-fleged programatic protocol handlers 

I'd like to drop here some [draft document](https://darobin.github.io/proto-handler-reqs/) of the idea that I introduced during one of the group meetings in the [TPAC 2023](https://github.com/w3c/webextensions/blob/main/_minutes/2023-09-11-wecg-tpac.md).

I hope it could help to continue the discussion, if you find it interesting.
