Feature Request: Implement ThreadEnvelopes for Notmuch Backend

[Lotram]

Hi,

Continuing with my himalaya + notmuch + gmail setup, I've noticed the notmuch backend is missing thread support in pimalaya-core/himalaya, despite notmuch having excellent built-in threading capabilities.

Would you consider adding this feature ?

If needed, I can try to implement it myself, though I'd need some guidance on how to contribute to this repo.

[soywod]

Indeed, I had no time to develop this feature yet. PR are welcomed, but I would rather wait: I am actually in the process of refactoring the core libs, and the next one I have in my TODO is the email lib (straight after releasing a new Himalaya version). Better to propose a PR on the new system as it should be easier to dive into. It may take a while tho, let's keep in touch within this issue.