feat: add background daemon

[ellie]

This will allow for several things

1. Automatic sync, not driven by shell usage
2. Keep database writes out of the hot path, which becomes an issue with certain filesystems
3. Allows us more flexibility
4. Potential for fully realtime sync

The downsides are that this is significantly more complexity. I've intentionally not included an automatic daemon start/stop here, and kept it opt-in. It will likely remain that way for a while.

I've also added a daemon feature. This allows you to compile atuin without this functionality on platforms that are not unix.

I've removed Windows from CI (for now...), as I cannot test or support it at this time. It should still build with the correct set of features. It also makes CI slow :(

Checks

• I am happy for maintainers to push small adjustments to this PR, to speed up the review cycle
• I have checked that there are no existing pull requests for the same thing

[atuin-bot]

This pull request has been mentioned on Atuin Community. There might be relevant details there:

https://forum.atuin.sh/t/moving-atuin-to-a-daemon/78/24